conv.py

#!/usr/bin/python
#
# Converted from https://gist.github.com/JonLundy/f25c99ee0770e19dc595 to Python3
import sys,json,base64,binascii
with open(sys.argv[1]) as fp:
    pkey=json.load(fp)

def enc(data):
    missing_padding = 4 - len(data) % 4
    if missing_padding:
      data += b'='* missing_padding
    return '0x'+binascii.hexlify(base64.b64decode(data,b'-_')).upper()

for k,v in list(pkey.items()): 
    if k == 'kty': continue 
    pkey[k] = enc(v.encode())

print("asn1=SEQUENCE:private_key\n[private_key]\nversion=INTEGER:0")
print("n=INTEGER:{}".format(pkey['n']))
print("e=INTEGER:{}".format(pkey['e']))
print("d=INTEGER:{}".format(pkey['d']))
print("p=INTEGER:{}".format(pkey['p']))
print("q=INTEGER:{}".format(pkey['q']))
print("dp=INTEGER:{}".format(pkey['dp']))
print("dq=INTEGER:{}".format(pkey['dq']))
print("qi=INTEGER:{}".format(pkey['qi']))

usage

# Let's encrypt converter
# Create a DER encoded private key
openssl asn1parse -noout -out private_key.der -genconf <(python conv.py private_key.json)
# Convert to PEM
openssl rsa -in private_key.der -inform der