I hope you're still checking this. I have an LC4 rolling chassis with no engine. Looking at used engines, the BMW 650 single seems to be much more available than LC4's (so, cheaper), and with a good record for reliability. They also have a right-side chain drive, although the engine mounts are quite different.
Do you have any idea if the Rotax fits in the LC4 frame? I'm thinking of going that route if I can do it with just some custom mounts.