There are many options, but few fit well, and even fewer are done well. I put a toyota 4age in my 69 Midget (sprite's twin if you weren't aware). You'll want to stay on the small side engine-wise if you want to make it work well. I'd go for lightweight and here are my favorites in order:
Suzuki swift aluminum 1.3 twin cam with samurai trans
A distant 4th would be the rotary for a couple reasons:
-they run hot and we don't have much room for a big radiator- also the car's interior is hot enough on any given day from march to october that you really don't want much more
-they run loud and we don't have much room for muffling. My 4ag is loud enough, and I understand that rotaries are pretty loud themselves
-Fuel efficiency is abysmal. We only have 6 gallon gas tanks. I get 35 mpg out of the 4ag and still struggle to break the 200 mile mark. Rotaries can get anywhere from 15-mid twenties, which really limits what you can do with them range-wise.
I've got a website with info on my build:
A guy named trevor has one about his Miata based build:
Another guy with the suzuki engine
I did the 4age 20v because pound for pound it offered the best performance- engine and transmission together weigh about 300lb and it puts out 150 hp roughly. Stock 1275 weighed about 260 lb, so it wasn't a weight gain that couldn't be overcome with some lighter bodywork, relocated battery etc.
Good luck, it's been a fun ride.