I'd be concerned about the plane being nose heavy and possibly over powered. It's built pretty light for an ARF, and the suggested 2 cycle power plan is a .61. I think the majority of them are coming out at around 6-61/2 lbs. I think that mounting it inverted would cancel out any tendency for a lateral imbalance, and the cowl would only need minimal cutting. I would go with a horizontal engine to avoid a high tank position and the associated idling and running problems, but since you already have an inverted muffler................................
Karl