Is there any modification for performance, the car, (looking for less consume fuel) for example, I can think of change to OBD1 and put a heating O2 sensor, or change a larger gearbox group (i dont know which one) thinks like this
Honda CRX D16A9 1989

 

header, exhaust, test pipe.
front lip
wiper/antennae delete, mirror delete.
Lower the car,
lighten the car
tune up

The mods you listed, aside from the gear ratio change would give marginal results.

 

You could try some of those low rolling resistance tires. Also if you convert to OBD1 like you are talking about, you could chip the ecu and have it tuned for fuel economy.