BinaryDriver,
First, welcome to RS25.
Second, take whatever you know about Honda swaps and throw them out the window. You cannot simply swap manifolds and make some minor wiring tweaks with Subaru engines like you can with Hondas.
Third, sell that EJ20G swap. Simply going for any "JDM Swap" in a Subaru doesn't cut it. The absolute headache that you are going to have to endure in trying to make this swap work will not be worth the end result. If you want OBD-2 compliance, which tells me that this is a daily driver/road-going car, you have to pay the price for it.
To try to make your "swap attempt" work:
For starters, you would need to get an EJ20G ECU wire harness, an EJ205 ECU Wiring harness...and then you would need to match ALL of the plug wiring on both sides of the ECU harnesses - EJ20G on manifold side, EJ205 on ECU side. Be prepared to find that the 205 harness has more items on both sides than the 20G. Also, you may find that there are items that were 2 or 3 wires on the 20G harness may be a single wire on the 205 harness. The EJ20G does not have a fuel pump controller system. The EJ205 ECU, however, will look for one. The EJ20G and EJ205 use different coils, too.
Then...after that, you're going to need to swap out the cam and crank wheels and sprockets to the 205 - completely different trigger pattern and timing. You might find that these won't fit. So now, you have 2 choices: Attempt to find a way to mod the heads to make them fit (may require LOTS of customizing work, like using 205 cams...and those don't fit 20G heads) - that won't be cheap (assuming the shop doesn't look at you like you've asked for D15 swap in an RSX), or buy 205 heads. Your next headache if you buy 205 heads will be the 20G manifold no longer fitting. So you can buy a 205 manifold to fix this issue, but now if you've modded the harness for the EJ205/EJ20G hybrid setup, the harness will not work.
Your swap nightmare does not end here. Assuming you managed to somehow get the 20G heads to accept everything that is needed for the 205 ECU to get it to start its own cranking process, now you'll need that multimeter, because it may not start. The engine may be looking for a specific ground where the ECU expects a 5v feed...or 12v burst. Or, thanks to design changes, the sensors are sending info that the ECU cannot read correctly. Good luck getting the new sensors to fit. And, that throttle body...
Your heart is in the right place. However, this sounds like the plan wasn't well researched before you started getting things together. We can go on for a long time here about this.
Get an EJ205 swap and wiring harness. Then you can simply pull your harness out of the TS and put the WRX wiring in without modding it.
