Well here are a couple easy quick ones: A engine chip, conforti and dinan make good ones as well as some other companies that I can't think of off hand. Catback exhaust is another. Those are the really easy things, I dont know much on the m20 so I'm not sure what you can do to get more power out of them.
You could also do shocks/struts and springs, that will make the e30 handle even better.