Well, if it's only against friends, it seems like you only need the stock engine...
So really I'd look around at a new gs15-r or rebuilding it...
Now, something to think about if you know some one in the trade, or can do it yourself... Do a engine port job... but becareful its easy to fark up, and you can't reverse it... but that will give you more power...
Also, when your going down that main straight, how long do you touch max revs for? like does the engine seem to not want to rev any further?
And maybe you should think about starting up a club, just put the word around of a club, and just try and keep running at the courts, charge a small membership fee yearly, maybe a $5 race meet fee, and save up and buy some track timing gear or something...
or just keep running how you are...
But let everyone know, everyone who works the club, volunteers too... no one takes money from it...