hi there,
if ur having a 1800cc fto... it's quite easy to get it modified. The engine code was 4g93, meaning that u can use the performance parts from mit 4g93 series. I'm pretty sure that most aftermarket parts such as Cam, Extractor, trottle body, ecu and etc are available in the market nowdays. you can even bolt on a turbo if u want to. But again, it's up to the level of mod that you would like to have under ur hood.
As for the most powerful version of FTO, i would say GPR (version R) > GPX > follow by others. GPR and GPX are using the same engine (Mivec 2000cc V6). However GPR has the 5 speed LSD transmission. On top of that GPR is lighter compare to GPX. If you're confused, just take GPR as type R (honda) while GPX will be the normal version of it. Again, lightened fto means that u will miss some stuffs in your car (most parts were analog)
Feel free to drop me a PM if u need more infos.
Thanks!