If your model has those hardwood or plywood motor mount rails that are built into the fuselage, you're better off mounting the engine as instructed. Modification is not impossible, but if you're not sure what you're doing to begin with, it's best to stick to the plans.

Most, if not all, commercially available engine mounts have a symmetrical mounting bolt pattern, which mean you can orient the mount in all 4 directions without redrilling holes on the firewall. Also the the thrust line typically stays the same.

Another thing I'll add is that while "side saddle" has many advantages, it isn't without drawbacks. Here are a couple:

1. Doesn't work well on models with cowl cheeks. You'll have to cut a big ol' hole.

2. With a conventional 2stroke glow engine, the side exhaust muffler hangs down at the bottom. This, IMHO, is worse than an inverted engine hitting its head on the ground on a nose over. I've seen many a muffler (including one of mine) mounted this way crack or break off on a hard landing.

I still think side mounting is ideal in many situations. Just didn't want the sidesaddle picture to get painted too rosey.