Satoh Beaver is an awsome tractor. I have a Beaver but have the engine disassembled due to an over heating issue due to being loaned out.
Anyway, I have disassembled the engine, transmission etc. These little tractors are built like tanks. Any problem I have had is a result of a sticking clutch or being overheated due to a leaking water pump. Both of these problems were due to operator error (1 mine an the other being loaned) I used mine to mow heavy grass (5 acres) for about 10 years. I bought it from the original owner who had purchased it new. When I disassembled the engine, there was practically no cylinder or crank wear.
I think these little engines were rated at 15 HP but the torque is awsome. I do not think I ever once stalled my tractor.