M10 Power up

Hi I have just sold a E30 323i and have a 318i that i am looking at modifying. Originally I was going to turbo it but it is too expensive. What is the most effective way of getting similar performance to a E30 323i/325i out of the M10? Looking at possibly a custom exhaust, cam & other external mods but dont really want to go inside the engine if possible... Any suggestions would be greatly appreciated.

Cheers.