When breaking in a new engine that is destined to be flown inverted, should it be mounted inverted on the break-in stand?

I had never thought about that. I don't think I would break it in inverted; never did before. However, if you can easily mount it inverted, maybe after you have run it a bit upright you might turn it over and learn something about starting it inverted.

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.