An engine doesn't care which way it's mounted. The operator though has to be aware of a few things. First, the centre line of the tank should be level or slightly below the centre of the carb barrel. With an inverted engine any fuel that drains from the tank through to the carb just dribbles straight down onto the ground. An inverted engine is more sensitive to the amount of priming because it gets trapped in the combustion chamber which can cause a hydraulic lock. Always turn the prop over a couple of times by hand before hitting it with an electric starter. A trap that's not always obvious with an inverted engine is that fuel can sometimes flow back through the pressure line into the muffler because the muffler is then way below the tank. The fuel in the muffler then gets through the engine's exhaust port and floods it. This seems to be the major problem in this case because of having to empty the muffler of fuel.