Better flowing exhaust, stronger spark, better fuel injectors, free flowing intake, oil cooler (mostly for longer engine/oil life), lower diff gears, etc. One could write a novel on what you could do to up performance.
Another idea is unless you want to keep it stock, for the amount of money you'll have in a rebuild/performance built motor, you may want to look into an M50 swap. Considering the cost/performance/labor/parts a swap may be afordable if you are trying to squeeze alot of juice from an M20 motor. Whole wrecked E36 are being picked up at public auctions for around $2,000-$3,000 it may be worth looking into.