you will have to convert to obd1 to use your cars electronics and be legal in 45 or so out of 50 states... putting an older engine than the car is highly illegal in a lot of places. check your local laws.
the tranny will cost you 500+ alone.
the b18b owns the b18a as far as head stuff and manifolds
its not that expensive either.
you will spend more on getting a b18a into a 5th gen then it would be to buy a newer, less mileage b18b