I wouldn't worry about power for the time being, spend your money on a good suspension set up, good brakes and some poly bushes. Get it handling and stopping first and then look to more power. It's no good having a million bhp but understeering off at every corner and having to stand on the brakes miles before anyone else.
Engine wise to start with I would say a good fluid service and freshen up all the ignition side of things, drill the air box and slap in a good panel filter. You'll be surprised how much difference that alone will make.
Head over to the track day/motorsport section of the forum as that should help you out loads.
Good luck and hopefully see you at a track day sometime.
Nick