Man, I get the money thing for sure. I have heard of people putting different engines in, I don't have any real details though. Your carbs though, are slide carbs and shouldn't be too difficult, cv carbs can really suck though.
If I were to try a different engine, the frame would probably work best with an i-4 engine. Though, I think an engine that isn't used as a stressed member would work better. Either way, it will be some work and you'll be welding and such. So, an i-4 600cc engine could probably be stuffed in without too much issue.
You'll end up needed all the electrics to run things.
The ride IS the adventure. The destination is just to get gas!