ECU Problem? Help appreciated.

So i have an '02 Celica GT which had the engine replaced with an '03 Celica GTS 2ZZGE M/T. There has been a bank one lean code ever since i bought the thing. I had to replace the ECU since it was for the 1ZZFE GT engine. After having tons of other mechanical problems (Vacuum leaks, sensors etc.) My ECU decided to throw a rich and lean code at the same time so my mechanic is starting to think i need either a new computer or a custom tuned one, i have tried a second computer but it's not any better and threw a bank one lean and a knock sensor bank one code right after turning it on and i haven't had it scanned since.

Also, with both computers i have a huge loss of power in for example second gear until i hit like 2.5-3k RPM then i get it all back, hopefully thats just the computer as well though.

If anyone has any experience/knowledge of stuff like this i would love to hear what you have to say, whether i can get another ECU that will work out or where i can get a custom tuned one etc. Thanks it advance!