RsaPem.cs

static class RSAParametersHelper
{
    public static RSAParameters FromPem(string pem)
    {
        using var rsa = new RsaPem();
        rsa.ImportFromPem(pem);
        return rsa.ExportParameters();
    }

    class RsaPem : RSA
    {
        private RSAParameters _rsaParameters;

        public override RSAParameters ExportParameters(bool includePrivateParameters) => throw new NotImplementedException();

        public RSAParameters ExportParameters()
        {
            return _rsaParameters;
        }

        public override void ImportParameters(RSAParameters parameters)
        {
            _rsaParameters = new RSAParameters()
            {
                D = parameters.D?.ToArray(),
                DP = parameters.DP?.ToArray(),
                DQ = parameters.DQ?.ToArray(),
                Exponent = parameters.Exponent?.ToArray(),
                InverseQ = parameters.InverseQ?.ToArray(),
                Modulus = parameters.Modulus?.ToArray(),
                P = parameters.P?.ToArray(),
                Q = parameters.Q?.ToArray(),
            };
        }
    }
}