For 5th gen chassi,
3sgte engine from Alltrac will fit in GT chassi without body modifications.
It's the best bang for $$$ if you are looking for some serious power.
Almost all 3sgte swaps retain FWD set up.
There are very few that have done AWD conversion with it.
It's not bolt on. It will take quite a lot of body modifications.
Straight-6 (J series engine) will not fit without serious modifications.
(cutting floor, firewall, etc...)
V6 (M or VZ series) will fit with some modifications.
It has been done on FWD (GT) and AWD setup (alltrac chassi)
V8 (UZ series) will also fit on Alltrac body with RWD conversion.
Do some search on 3sgte swap.
A good place to start:
www.celicatech.com
If you want to avoid engine swap, you can also turbo your 5sfe engine.
For small budget mod, consider intake, exhaust, advance timing, etc..
They will not take you very far though.
May be 10-20 hp gain.