Ooooo! Sounds like time for an engine swap! 🙂
It's never too late to put a nice V8 and a 5-speed in that Mini. I'm sure a 289-302 Ford will fit. maybe even a small block Chevy; not an LS, that's just not right, but a nice 283 or even a 327.
Given the size restrictions, I'd probably opt for the Ford, even though I'm not really a Ford guy, they are a bit smaller in envelope that most other V8's for the swap. 🙂
Love the car, especially the paint color!