I think your Fox .50 might be touching 10,000 rpms or exceeding it a little as it limbers up some more.

My Fox .45 diesel conversion turned a Master Airscrew 12x8 Prop at 9,400 rpms. It even had a muffler on it that is considered very restrictive on a glow engine but on a diesel it wouldn't be a issue. But I think it might do better as it limbers up more as it is a new engine.
This is a NOS early model Fox .45 engine from when they first came out. At the time Fox had some rather small mufflers for them. But the muffler is maybe perfect for a diesel engine.