A hole in the cat isn't going to effect it, who tells you this? Besides the cam sensor is set with the engine off. It is possible the cam sensor could be bad but I wouldn't pull it until that's been verified. There could be multiple issues that have just built up over the years which the computer can no longer compensate for. I would check the driver side header for cracks, check all the ecm fuses (they've been known to blow and still let the car run, but run like crap), get the tps set correctly for now. Don't know what brand of 02 that was put in but do not use bosch, use denso or acdelco. Replace all the vacuum lines and be done with it. This guy is gonna have to spend some money no matter what. I highly suggest you have him buy a plug for the winaldl program. It will be the best 30 bucks he ever spent. You will need this to verify maf operation, blm, iac counts, tps (to see what the ecm sees, not what the voltmeter sees), cross counts, block learn integrator (ecm relearn), 02 at idle and wot, etc. For now I would also make a block off plate for the egr just to make sure it isn't causing a vac leak. Make sure you leave the rest of the stuff hooked up though and plug the vac lines.
All the cam sensor does is tell the injectors when to fire after start up. You are going to have to reset this before doing anything. The reason the "veteran mechanic" couldn't fix anything because he didn't know what he was looking for, and most don't. obd1 may be less sophisticated than obd2 but you still need a scan tool and base scan values to get things right. Little things are what makes the difference on an FI car that runs, and one that runs "better"
I got my homemade cable from this guy. He does a good job
http://www.fiero.nl/cgi-bin/fiero/showThread.cgi?forum=4&thread=041052&style=printable
I don't know if he still makes them or not but it's worth a shot. If you do get one from him you have to bend one prong down otherwise it will go into diagnostic mode at all times.
Here's the program you can download.
http://winaldl.joby.se/ there's also instructions on how to make your own cable but I'd just buy one if possible. You will also need a 9pin to usb adaptor unless you have an older computer. THese adaptors can be had off ebay cheap and they work.
You keep wasting your weekends doing it your way and putting things out of whack even more than what they are or you can fix it the way I suggested. Yes I do have things I could be doing now but since I'm such a nice I guy I decided to post all this to help you get that car running. It may be a good idea to have the owner get in on some of this also, it will be nothing but beneficial for him.