more power to tsikot

I'm a new car owner, i have a 91' galant gti 16v 4g63 engine
i had the year for about a year now.. and is experiencing a couple of issues lately

i observed that my car is loosing its power,

1.) rpm is dropping badly specially when the ac, lights and stereo is on..
2.) the engine sounds like a diesel engine, and i saw some oil around the spark plug
3.) oil seals under the engine are leaking and the smoke coming out from the exhaust smells bad..

i believe i need a major repairing to do with my car..and badly needs help cause i don't know much about cars yet.

is my engine for a total overhaul and how much would it cost?
and do you know any good mechanics for this kind of car

hoping to hear from you guys.. badly needs help/


thank you!