Given an exponent and modulus, output a PEM encoded public key.

MakePublicKey.java

import java.math.BigInteger;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPublicKeySpec;
import java.util.Base64;


public class MakePublicKey {
    public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeySpecException {
        var modulus = new BigInteger(args[0], 16);
        var exponent = new BigInteger(args[1], 16);
        var spec = new RSAPublicKeySpec(modulus, exponent);
        var pub = KeyFactory.getInstance("RSA").generatePublic(spec);
        
        System.out.println("-----BEGIN RSA PUBLIC KEY-----");
        System.out.println(Base64.getMimeEncoder().encodeToString(pub.getEncoded()));
        System.out.println("-----END RSA PUBLIC KEY-----");
    }
}

Note that as of java 11, you can run this like java MakePublicKey.java MODULUS EXPONENT