Good engine conversion for FB? Not rotary

Recently I found a 1980 RX-7 FB/SA in really good condition, and by good condition I mean it has little to no rust. Just a bad interior and paint. Body is in good condition, engine still there with no leaks. Didn’t get to start her up as the owner removed the wheel lock cylinder, but it looked like it could run without much hassle, and the owner is selling it for 700. Now this seems like a really good deal and all but there’s one problem; I don’t want the 12A that’s in it nor do I want to swap a 13B into it. I was hoping to find a suitable 4 cylinder swap for the car, maybe a Miata engine, but people say those are too heavy, or an SR20, but all the research I’ve done has led to hearing that its stupid and that I’m stupid for swapping out the rotary.

If I can’t find a good swap then I’ll just restore the car and sell it, but I want to be able to keep it for myself. Thing is, I want good mpg, don’t really care much for power, just enough to still have fun with. I’m really hoping that I can find a swap or a solution to this because I really want the car for myself. Otherwise I’ll just restore it, sell it and buy an FC to do an engine swap with or look for a BMW E30.

So I’ll ask the question again; is there a good engine conversion for this car that would get me good mpg?

Thanks

P.S. : Please don’t hate me for wanting to put a piston engine into this