That YS engine was an excellent engine in its day but that day was almost 30 years ago. Trying to get parts for it would probably be almost impossible today so I'd recommend a new engine. I agree with others, the OS 46 is a good way to go and will do well with a large diameter, low pitch prop your project will probably require. A 2 blade prop will be more efficient than a 3 blade especially in the diameter range that engine will be able to turn.
I 100% agree, find a pilot that understands flying a high wing loading, slow airframe. Your project is way out of the range of what a typical model would fly like. Good luck and please report back how things went.