Ok, the sender is easy to remove, but what is involved in removing the rest of the OPG setup on a 91?

 

Pull the hood, the gauge cluster, remove gauge from plastic housing.
Atleast that's how it goes on my '99. Like 4 screws hold the cluster in. 3 screws hold the gauge itself on the circuit board in the housing. Simple stuff.

 

Do I need to pull any wiring or is the 1.8 the same wires?

 

Not sure on that one buddy. Mine only has one wire going to it though.

 

It will function correctly without any wiring changes. Just do as recommended above and swap the gauges.