The best performance mod you can do to it is swap a dohc in. But you can still do some stuff, warm air intake, geo throttle body, believe they do make a header for the sohc. Some nice wires, leave the stock coil packs in they work great just make sure they are nice and clean. Upgrade / add some extra ground especially one to the coil packs, there are four bolts that hold it to the tranny add a ground to one of them.
If its a manual trans you could add a short shifter for the performance feel (unlike stickers does not add hp).
Stiffer motor and trans mounts. (does not add any, but helps put what hp you have to the ground).
Thats a few things anyways..