well me being an American, I have no idea what the Excel is, but I can still make a few comments about the engines. Is the Excel similar to our Mitsu Mirage, being a front drive sedan (I hope that's a good guess)? In that case, your motor choice is simple: the 4G63 (a 2.0L inline 4, turbocharged). That's the motor in the famed Mitsu Lancer Evo's, and the one in our older domestic market Eclipse, Talon and Laser turbo models. It's a gem of a 4 cylinder, with seriously big power only a turbo upgrade and some engine management basics away. The version in our Eclipse Turbo's has 195-210hp stock (depending on model year), but is capable of putting out over 400hp without any internal modifications. And what's more, if your car is based on the Mirage/Lancer platform, there is a TON of aftermarket equipment ready to make it a beast. The motor itself also has transmissions available for front wheel drive, which is a big plus over the VR4 motor ( a 3.0L V6, twin turbocharged) that has never to my knowledge been used on anything but all wheel drive. That engine is also much more expensive and heavier than the 4G63.
For information reagarding the performance of the 4G63, check out
http://www.buschurracing.com/ and
http://www.extrememotorsports.com/ You should find enough information there and in those site's links to answer all your questions.
Hope this helps, but if I'm off base with my guessing game I apologize. And glad to see some folks from Australia around here!
Ps- What's a Gemini?? Damn auto manufacturers just have to name EVERYTHING in Au different from here.