I've broken in YS (.60 2-stroke and 1.20 4-stroke), OS (.25VF, .48 Surpass, BGX) and Enya .53 4-stroke engines inverted without a single problem. All ran beautifully out of the box and were easy to tune. Install the tank properly, and the engine won't know, or care, what direction the cylinder is pointing.

just my .02 and experience,
John