The only problem an "upside down" engine presents is getting it started without drowning the glow plug. One can also flood the engine easily and cause hydraulic lock (combustion chamber fills up with fuel) - definitely not good. To solve the problem you can get someone to hold the plane in a normal upright attitude while you start the engine - or you can put it in a cradle. Once started, the engine will run just fine.

The alternative is to install the engine horizontal being careful on how the muffler orients. Remember the centerline of the tank should align with the needle valve. You may have to adjust this. With a horizontal mount at least the plane looks good from one side . A proper horizontal mount doesn't require any gymnastics to start the engine and also avoids the dreaded hydraulic lock.

This plane flies great. It is fast and stable. You will enjoy it.