I'm proud to be on a LeMons/Chump team that runs an "Mx-7" - it's a 1988 Gen2 Rx-7 with a 1.6 liter Miata motor. The car is quite slow, easy to drive, and gets great mileage. Add the latter to the 16gal(?) stock tank and we rarely refuel. But it begs for more hp.
The extra 1" length of the 1.8 would be a problem. Don't know how either would fit in a Gen1 Rx-7.