There are literally a ton of options but mainly for the water-cooled T3s vanagons. Really depends on either how talented and resourceful you are OR how how deep pockets are! The option list for aftermarket engines and trans changes yearly. A good example is now there is a kit to retrofit a Sooby 5 speed trans into a T3. Pretty cool stuff. Spend a night or two searching these forums as well as the ones at the samba. My preference is the samba ( no offense to anyone here) there's more traffic there.
Engine options to search:
- turbo diesel (IDI)
That's just off the top of my head. I'm sure there's more.