Merge of Brad Smith's nsfe_to_nsf2.py and nsf2_strip.py scripts with a fancy argparse interface slapped on it. I needed this to downgrade an NSFe file far enough so nsf2midi would recognize it. Putting this here in case anyone else has the same problem.

nsfe_to_nsf.py

#!/usr/bin/env python3

import sys
assert sys.version_info[0] >= 3, "Python 3 required."

#
# nsfe_to_nsf2.py
# Brad Smith, 2018-08-24
#
# This converts an NSFe file to a preliminary "NSF2 with metadata" format.
#

import argparse
import struct
from os.path import exists, split, splitext, basename, join

def nsfe_to_nsf2(nsfe, show_chunks):
    # NSF structure to fill in
    nsf_header = bytearray([0]*0x80)
    nsf_data = bytearray()
    nsf_suffix = bytearray()
    info = False
    data = False
    # build default NSF header info
    nsf_header[0:5] = b"NESM\x1A"
    nsf_header[5] = 1 # version
    nsf_header[0x6E:0x70] = struct.pack("<H",16639) # NTSC speed
    nsf_header[0x78:0x7A] = struct.pack("<H",19997) # PAL speed
    # parse NSFe header
    if len(nsfe) < 4:
        raise Exception("Does not contain header.")
    if nsfe[0:4] != b"NSFE":
        raise Exception("Does not begin with 'NSFE' fourCC.")
    nsfe = nsfe[4:]
    # parse NSFe chunks
    while len(nsfe) > 0:
        if len(nsfe) < 8:
            raise Exception("Malformed chunk, does not have 8 bytes for size/fourCC.")
        size = struct.unpack("<L",nsfe[0:4])[0]
        fourcc = nsfe[4:8]
        if show_chunks:
            print ("'%c%c%c%c' (%d bytes)" % (fourcc[0],fourcc[1],fourcc[2],fourcc[3],size))
        if (size+8) > len(nsfe):
            raise Exception("EOF reached in the middle of a chunk. Incomplete file?")
        # parse chunk
        raw_chunk = nsfe[0:8+size] # chunk with header
        chunk = raw_chunk[8:] # chunk without header
        if   fourcc == b"NEND":
            nsf_suffix += raw_chunk # append this chunk
            break # stop parsing here (NEND signals end)
        elif fourcc == b"INFO":
            # parse NSFe INFO
            if size < 9:
                raise Exception("INFO chunk must be at least 9 bytes.")
            nsfe_load = chunk[0:2]
            nsfe_init = chunk[2:4]
            nsfe_play = chunk[4:6]
            nsfe_reg = chunk[6]
            nsfe_exp = chunk[7]
            nsfe_songs = chunk[8]
            nsfe_start = 0
            if size >= 10:
                nsfe_start = chunk[9]
            info = True
            # fill NSF header
            nsf_header[0x08:0x0A] = nsfe_load
            nsf_header[0x0A:0x0C] = nsfe_init
            nsf_header[0x0C:0x0E] = nsfe_play
            nsf_header[0x7A] = nsfe_reg
            nsf_header[0x7B] = nsfe_exp
            nsf_header[0x06] = nsfe_songs
            nsf_header[0x07] = (nsfe_start+1) & 255
        elif fourcc == b"DATA":
            nsf_data = bytearray(chunk)
            data = True
        elif fourcc == b"BANK":
            for i in range(0,min(8,size)):
                nsf_header[0x70+i] = chunk[i]
        elif fourcc == b"RATE":
            if size >= 2:
                nsf_header[0x6E:0x70] = chunk[0:2] # NTSC rate
            if size >= 4:
                nsf_header[0x78:0x7A] = chunk[2:4] # PAL rate
            if size >= 6:
                nsf_suffix += raw_chunk # append to pass Dendy rate
        elif fourcc == b"NSF2":
            nsf_header[5] = 2 # version
            if size > 0:
                nsf_header[0x7E] = chunk[0] # pass NSF2 bitfield
        elif fourcc == b"auth":
            append = False
            ci = 0
            oi = 0
            for ci in range(ci,size):
                c = chunk[ci]
                if c != 0:
                    if (oi < 31):
                        nsf_header[0x0E+oi] = c
                        oi += 1
                    else:
                        append = True
                else:
                    break
            ci += 1
            oi = 0
            for ci in range(ci,size):
                c = chunk[ci]
                if c != 0:
                    if (oi < 31):
                        nsf_header[0x2E+oi] = c
                        oi += 1
                    else:
                        append = True
                else:
                    break
            ci += 1
            oi = 0
            for ci in range(ci,size):
                c = chunk[ci]
                if c != 0:
                    if (oi < 31):
                        nsf_header[0x4E+oi] = c
                        oi += 1
                    else:
                        append = True
                else:
                    break
            ci += 1
            if size > ci:
                append = True # ripper name as well
            if append: # these strings won't fit
                nsf_suffix += raw_chunk
        else: # other/unknown chunk
            nsf_suffix += raw_chunk # append the chunk as-is
        nsfe = nsfe[8+size:] # next chunk
    if info == False:
        raise Exception("No INFO chunk found?")
    if data == False:
        raise Exception("No DATA chunk found?")
    # data length, offset to NSFe suffix
    nsf_header[0x7D:0x80] = struct.pack("<L",len(nsf_data))[0:3]
    return nsf_header + nsf_data + nsf_suffix

def nsf2_strip(nsf2, show_cut):
    if len(nsf2) < 0x80:
        raise Exception("NSF2 too short for header?")
    nsf = bytearray(nsf2)
    version = nsf[5]
    nsf2_bits = nsf[0x7E]
    if version >= 2 and (nsf2_bits & 0x80) != 0:
        raise Exception("NSF2 metadata is flagged as essential.")
    suffix = nsf[0x7D] | (nsf[0x7E]<<8) | (nsf[0x7F]<<16)
    nsf[0x7D] = 0
    nsf[0x7E] = 0
    nsf[0x7F] = 0
    stripped = 0
    if suffix != 0:
        cut = suffix + 0x80
        if cut > len(nsf):
            raise Exception("NSF2 metadata pointer is past end of file?")
        stripped = len(nsf) - cut
        nsf = nsf[0:cut]
    if show_cut:
        print("%d bytes stripped." % stripped)
    return nsf

def main(args):
    for f in args.file:
        try:
            nsf_dir, nsf_base = split(f.name)
            nsf_name, _ = splitext(nsf_base)

            if nsf_name + ".nsf" == nsf_base:
                print("Error: Input file is output file.")
                continue

            new_nsf_name = join(nsf_dir, nsf_name + ".nsf")

            print(f"Reading {nsf_base} ...")
            nsfe = f.read()
            nsf2 = nsfe_to_nsf2(nsfe, args.debug)
            nsf = nsf2_strip(nsf2, args.debug)
            print(f"Saving {basename(new_nsf_name)}...")
            open(new_nsf_name, "wb").write(nsf)
            print("Done.")
            print("-" * 25)
        except Exception as e:
            raise e

    print("Done with all files.")

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="Specify one or more NSFE files to downgrade. The resulting .nsf files will be written next to the .nsfe files and will overwrite any .nsf files that happen to already exist (unless the new file's name is the same as the input file's).")
    parser.add_argument("file", type=argparse.FileType("rb"), nargs="+")
    parser.add_argument("-d", "--debug", action="store_true", help="Show more information")

    args = parser.parse_args()

    main(args)