Upright will definitely be easiest.

But if you wanted to mount it sideways, that MIGHT be ok. Check to see if your muffler will clear the firewall, and keep in mind that you'll have to rebuild the nose to close it in.

Personally, I always try to avoid mounting inverted. There is some controversy as to whether or not engines have problems running upside down, and I've had some that work well and others that didn't.

But a good reason for beginners to avoid it is the fact that if you nose-over, your engine's head is the first thing to take a hit.

But, the 4* is a proven design. Until you have a little more experience under your belt, I would suggest you stick with what works.