I like to stick to the MFG's plans as much as possible, besides, a REAL cub has heads sticking out of the cowl. I have also had some 4-strokes run great inverted, and some run poorly for no apparent reason.

But the Main reason I don't like to invert an engine (Especially a 4-stroke) is that if you nose over, you grind down the Rocker arm covers (Or worse)

Stick it out the side, you'll never see it at 500 feet.