By the early nineties emission control was built into the fuel control system and the only add on devices were the EGR and the air pump, which is only active during the first few minutes of operation before the fuel control system goes into closed loop operation.

The air pump can seize and is very expensive to repair or replace, so one thing you could do is fit a shorter accessory drive belt to bypass the pump, and you could also remove it but that might generate an error code and light the CEL.

Disconnecting the EGR control system might also light the CEL, but I don't think it would be a problem to simply block the passage at the coldest point in the system that you can access, and you will need a plug that can withstand high temperature, like steel.

The OE exhaust system is actually quite efficient even with the catalyst and is not worth modifying IMO.

By far the biggest (and easiest) improvement you can make for better fuel economy and more power is to remove the R16/1 resistor. This will quicken the rate of spark advance with engine revs resulting in noticeably more low end torque/power and better around town fuel economy.

For emission testing I short the R16/1 socket and replace a short piece of rubber tube in the vacuum advance signal line that is plugged. This considerably retards the entire spark advance map, which raises EGT and lowers measured emissions. The car can barely get out of it's own way below 1500-2000 revs, but my emission test station is only a couple of miles away, so I don't have to drive it in that gutless condition very long.

You can search for posts started my me, keyword R16/1 resistor, that have all the test data. Further searches will reveal the location of your R16/1 as it varies with model.

Duke