A transaxle is just a transmission that includes a differential and maybe space for a clutch package as well. That said, the 914 is a very flexible car when it comes to mods. Lots of space to do it.
The mods from one transaxle to another usually have to do with the ring and pinion flip (or running it upside-down) if it comes from a front wheel drive car, how to shift it, how to bolt it up to a motor and how to attach the axles... and sometimes how to keep them cool.
Dude, you can do ANYTHING. It depends on how handy you are and how much money you are willing to spend for the stuff needed to build what's in your head. The more labor you do yourself the better.
For many of us it's not how much power we have at our disposal but how a particular powerplant will change the inherently wonderful handling characteristics of a mid-engine car. Low is good... lower is better.
Personally I like GOBS of power and I like it low, so I stick to flat sixes. But I've driven outstanding and quick 4's that almost took my breath away. I'd have a good four in a minute.
They're so light that the ability to flick it through a turn is paramount. I want to feel the skin on my face pulling sideways in turns... yeah, that's it.
Good luck with your search for the right combination!