RE: "50" size planes
The .50-size two stroke is actually a pretty popular engine size:
O.S. Max .55 AX
Kangke SK .50A
JBA .56A
Magnum XLS .52A
Super Tigre GS-51 Ringed
Evolution .52NX
These are just examples off the top of my head, I'm sure there are more. Most ".50-size" ARFs are designed to run off a ball-bearing .46 2-stroke or .70 4-stroke, but are too heavy or too high-performance for a plain .40-sized bushing 2-stroke or .52 4-stroke to power them with the authority that the pilot will want.