Engine oil leaking

My 91 4 door GS Integra shakes violently when it gets warmed up after I have been driving for about 45 minutes in the street on regular Drive and in Park but it stops when I shift to neutral what could be causing the shaking?
A used engine was put in recently and is leaking oil but I keep filling it up so that it always has some oil, maybe a bolt is not tight or the cam seal is bad. Where can I go so that it does not cost that much to fix the cam seal in the engine? How much???$$$

I don’t have an auto tranny, so take it for what it is, but when the engine was swapped was the torque converter in good shape? Sounds like that’s the problem but once again, I may be full of sh1t. :wink:

Tha cam seal on the end near the distributor is like $10 from the dealer and it can be popped up and popped in quite easily.

Hollar back if you need to know how.