Convert the AC to ascending, use the rest of their stats straight-up; weak-ass hp/dps and all. Stat out the BBEG to be overwhelmingly challenging in your edition of choice. Enjoy your extended adventure-day. Easy peasy.
Not monsters but, a lot of old school AD&D adventures describe locks/trips/traps in great detail in order to allow players to figure them out without resorting to incredibly difficult Thief % rolls. For full flavour and immersion, you should encourage your players to figure shit out similarly.
Old school feel. New school rules. Don't let the grogs discrourage you.