The engine doesn't care which way it's mounted when it's being run in. But Jim had a good point, if you can mount it inverted in the stand then give it a try after it's finished running in. You'll be familiar with the engine by then and it'll give a little practise at learning how to prime it because then the fuel has to run uphill from the carb into the engine. Make sure you lower the tank too so the centre of the tank is still level with the carb barrel.