Well you could get yourself a nice new exhaust. Get an aftermarket header, wrap it in header wrap, do a full madrel bent exhaust 2.5", High flow cat, one or two resonators (depending how quiet you want it to be), and a new muffler.

You're looking at about $700 US there.

Get yoursef an Intake, Adjustable cam gear, and colder plugs, You're looking at another $200.

Lastly a Nitrous kit. Depending what you buy there between $400-$600.

You'll be plenty fast then. The H22 is a well built strong engine and is great at holding Nitrous stock. You can spary a 75 shot without worry.

All those prices are for new products you can probably save money by buying some used, some aftermarket brands that aren't as popular as the large tuners would also be a less expensive alternative.

You've already got a sweet car with some nice mods and your definately faster then stock and since you have a H22 there is much more head room to play with.

Good luck with whatever you decide.