What about a boxer engine from a Subaru? It's been a good 2 decades since I last tinkered with an RX7 but if I recall hood height is a premium. I don't remember the available width, however.

A WRX engine puts out 300hp or so. It'd be a real rocket! Even in NA form it would certainly scoot it.

That said the real joy of driving an early RX is the lightweight feel of the car. Maybe you could find a modern rotary from a new RX? They put out a lot more power than the old ones.

Found a newer one here: https://cpprohomeoh.car-part.com/image?seller=3519&partGUID=3519-1-147607&vehicleGUID=3519-1-V1744&display=2009%20Mazda%20RX8%20Engine%20Assembly-Stock%23%20002534

Granted it'll cost you a pretty penny once all done but it might be the same or less than another swap and would keep it in the family, retain the balance of the car, and be far more reliable than the old RX-7 power plants were (apex seals, if I recall, can be replaced on these).