First make sure the motor is healthy....have it checked...compression, etc. Adding mods to a tired engine will usually stress it and force it to go south even quicker.
If its healthy...things like a cam, bigger carb or dual carbs, Fuel injection, exhaust, pistons, etc....ALOT can be done to this engine...just depends on how much power you want. Remember the more power you add, the less streetable the car will become. If you want 200 hp, It will idle rough, not have alot of low end grunt, and be genarally not fun...until you get it where you can rev it and it will sing to 9k rpm....so if you want ALOT of power, look into another car or an S14 (M3) engine which will have 200hp stock..but the engine alone with 150k miles on it may cost you 5k...and another 5k to install..and another 3k if it needs a rebuild......if you want 150-160 hp, then it can be done with this engine and still be very driveable...