I have a st165 in kinda iffy condition, I was wondering since it has a usdm wiring harness and all, what would be the main problems in swapping the motor and such into a st204?
Sorry if this is the wrong forum, and I have tried looking around, the alltrac.net forums won't let me view theirs saying "You are not authorised to read this forum."

the ST165 is a 1st gen 3S-GTE. It will be a wiring nightmare to get it to work. short answer, yes it will fit and bolt in fairly easily. Long answer, it will take a bit of time.

I hope you mean ST205. It would be pointless to swap in an N/A engine..

I would be swapping out my 5sfe for the 3sgte
and the 3sgte is a jdm 2nd gen that the previous-->previous owner had installed

even the 2nd gen is a wiring nightmare to install in a 6th gen celica. yes it can be done, you will have to build an entire harness basically. the difference are huge between the cars.