I personally haven't had any problems mounting an engine sideways. Seems to run well. Now inverted I have before. Especially with 4-stroke engines, it's a good idea to use a remote glow system when mouting an engine inverted. I have one plane with the engine mounted sideways, just to be safe I have remote glow set up on it as well. It's up to you, but I haven't had any problems with sideway mountings on both my 2 and 4 stroke engines.