Could be a combination of things. You mention the car is modified, so it could be modified to the point it needs a tune, especially if the cam is quite aggressive.
Could also be a combination of lack of converters as well, or a faulty sensor. Could be a vacuum leak.
Since this car was your brother in laws, maybe just maybe he kept receipts for parts he bought.
You might ask other family members if they have seen any paperwork. Maybe talk to his friends,they probably know what he put in it.
If all else fails get it tuned by a reputable shop.
A switch back to what? 19 lb injectors is a start
This gas thing is getting to be just like 1983 (use only what you need)
Think dual cats and no mufflers. We are all in this air thing together
must know what's there 1st. Then down a ck list on each item. Not throw $ at every lill guess (that can burry the actual problem even more). I like the idea of getting a "changed list" from the 'builder'. /Or/ get out the wallet'n turn it over w/car to original guy. If U grow some U tell him he *ucked the car & to fix it correctly the 2nd time - he sure didn't the 1st (even tho a deceased relative did pay)...
I just bought a 90 lx and the exhaust smell was awful . If you drove down to the store and back you would reek of exhaust . I started cheap and changed the thermostat and that cured my problem . The car was not coming up to temp so the computer was staying in cold mode and dumping fuel constantly.