[Tinker51]

Ummm, I cannot imagine any of the advantages you have mentioned so far as being enough to justify the engineering effort involved. If you are worried about sparkplug access then build a small block car. You can almost stand in the compartment. As for lowering the CG, go with alum heads. Hydraulic lock would be a very real problem. I never start a round aircraft engine without running the engine through a complete cycle either by hand or on the starter only (depends on the type of aircraft and engine) before adding fuel or spark. You can cause some very expensive damage in a big hurry if you just start one up and it is locked. For sure inverting it would be cool, but it just seems like there are easier ways to overcome the problems you have posted.