If you want 250 whp in a miata, you're going to want a 1.8 unless you're buying a car that's already built.

If you go with a 1.6, there are a good number of things you're going to want to change, like the differential for example. The diffs on 1.8 cars can handle up around 300+whp.

V8 swap kits cost around 4 grand, and you're going to have to add the cost of the engine and transmission on top of that, as well as numerous other random costs.