Code:

import os
import time
import base58
import binascii
import secrets
import secp256k1 as ice
from time import sleep

os.system('clear')
t = time.ctime()
print(t, "GOOD LUCK AND HAPPY HUNTING...")

address="1Myn5hoo8pghPZBTTibLjoksVH1k1Fgxmd"
known="L4cgbejhcrqGVwC7qL3owVJSS87zJm53JCpoFokX55"
while True:
alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
unknown = ''.join(secrets.choice(alphabet) for i in range(10))
WIF=(f'{known}{unknown}')
if WIF.startswith('5H') or WIF.startswith('5J') or WIF.startswith('5K') or WIF.startswith('K') or WIF.startswith('L'):
if WIF.startswith('5H') or WIF.startswith('5J') or WIF.startswith('5K'):
first_encode = base58.b58decode(WIF)
private_key_full = binascii.hexlify(first_encode)
private_key = private_key_full[2:-8]
private_key_hex = private_key.decode("utf-8")
dec = int(private_key_hex,16)

elif WIF.startswith('K') or WIF.startswith('L'):
first_encode = base58.b58decode(WIF)
private_key_full = binascii.hexlify(first_encode)
private_key = private_key_full[2:-8]
private_key_hex = private_key.decode("utf-8")
dec = int(private_key_hex[0:64],16)

HEX = "%064x" % dec
wifc = ice.btc_pvk_to_wif(HEX)
wifu = ice.btc_pvk_to_wif(HEX, False)
uaddr = ice.privatekey_to_address(0, False, dec)
caddr = ice.privatekey_to_address(0, True, dec)
if caddr.startswith('1Myn5'):
t = time.ctime()
print('Time : ', t)
print('PrivateKey (hex) : ', HEX)
print('PrivateKey (dec) : ', dec)
print('PrivateKey (wif) Compressed : ', wifc)
print('PrivateKey (wif) UnCompressed : ', wifu)
print('Bitcoin Address Compressed = ', caddr)
print('Bitcoin Address UnCompressed = ', uaddr)
print("\n continue...\n")
if caddr == address:
t = time.ctime()
print("Winner Found!:",t, caddr, wifc)
f=open('winner.txt','a')
f.write('\nTime: ' + t + '\nPrivateKey (hex): ' + HEX + '\nBitcoin Address Compressed : ' + caddr + '\nBitcoin Address UnCompressed :' + uaddr + '\nPrivateKey (wif) Compressed : ' + wifc + '\nPrivateKey (wif) UnCompressed : ' + wifu + '\n==================================')
f.close()
sleep(4)
break

Here is the script. It's fast as hell on my machine.
An example.
Address: "1Myn5hoo8pghPZBTTibLjoksVH1k1Fgxmd"
Part of the key "L4cgbejhcrqGVwC7qL3owVJSS87zJm53JCpoFokX55"

Last 10 characters are missing - about 5 seconds - or was this a lucky combination. Grin