If your wanting to keep it a 4 cyl, the best bang for your buck would probably be a turbo swap/build. Easiest way to do it would be to find a thunderbird turbo coupe (87-88 are the best years for it) donor car and swap everything into the stang. Then you pretty much have an SVT
. Internals are forged, they handle boost really well for a stock engine. It is not unheard of to get 200 rwhp out of a stock engine with stock turbo. Support in the parts department are limited to do-it-yourself if you have machining capabilites and fabrication experience or very high priced one off items from a couple companies. Stock engines can handle 15-18 psi with stock turbo. If you want to get crazy you can use volvo head, turbo, intercooler with some modification and can push over 20 psi. There are those out there that are running 300+ rwhp out of there 2.3L turbo. My next project after my current GT will be an 85-86 turbo 4cyl. turbotbird.com will give you a ton of info to go through. as well as turboford.com Good luck and keep us posted.
ps, I had an 88 Tbird turbo coupe and loved it. If I would have been smart I would have found a mustang coupe body and swapped everything into it.