If you are referring to the 4 cylinder engine, then the answer is yes. Both 89 and 90 use the 3S-FE 2.0 4 cylinder engine. If you have the 2VZ-FE 2.5 V6, then you cannot swap from one to the other.
YES!
Yes it is and its the easyest to do.
if you mean the hole at the bottom of the bell housing, where the trans. bolts to the engine, there are 2 things. a leaking trans. pump seal or trans. pump O ring. to fix it in this case the transmission and the torque converter have to be removed. the pump unbolted and pulled out of the housing, then the seal and o ring replaced.
Right hand end (as you're facing the engine) of the engine block, above the distributor.
The easiest engine swaps for a 240sx would be an sr20det or a ca18det as these require no modification to the motor mount location.
The throttle body intake will not bolt to the Magnum engine block.
for manual trans. it is behind the steering wheel, for automatic it is on the left side of engine compartment near the engine.
no its a different engine so it wont match up to the tranny preludes are 2.0s and 1.8s civics are 1.6
You could, but because the '94 Prelude is heavier than the '89 Prelude, it doesn't make sense to swap in a smaller powered motor.
Yes
It's possible to swap in a B20 series motor from a 3rd gen Prelude to a 5th gen Civic. However, it would be more practical (and easier to swap in) a D-series motor which there are multiple variants for the Civic between the late '80s into the '90s.
Yes, but, you will have to defeat the computer, which will probably negate your local smog laws.