I'm really curious. Is this a single cylinder tractor? Gas or diesel? It appears that the engine is mounted transverse - that is with the drive shaft oriented left to right rather than front to back. Is that true?
If Yanmar was consistent, then it should be about 20 engine hp. I cannot tell whether it has a pto or not - but pto hp would be around 17.
Good luck! It looks like you got a unique tractor.