Gear oil leak problem

I do not have information on your model, but your repeated problem suggests the source of the leak is not easy to determine or is hidden. If this started right after the engine was replaced, a good suspect is the front pump seal. It is common for them to become hardened with age and lose their flexibility. When the engine is removed, gravity pulls the torque converter down, and that flexes the seal way beyond what is normal and can crack it. You will find the fluid leaking from between the back of the engine and the front of the transmission, where they bolt together. It is common practice to stick a pair of wedges under the torque converter to hold it up just before the engine and transmission are separated.

If the leak is real slow. A good approach is to wash and dry the area, then add a small bottle of dark purple dye to the transmission fluid. Search a little while later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Auto parts stores will have the dye for the fluid being tested, and those that rent or borrow tools should have a black light.