I built a Young engine many years ago, I think it was my first project form a casting kit. I bought the kit from someone at an engine show, not from Young. I probably still have the plans in my file drawer. I recall they were large sheets and several pages.
I had a lot of trouble getting the valves tight, but that was a problem of my inexperience not a fault of the design. I was thinking the other day of dragging the engine out and seeing if I can get it to run better now. It has been under my bench many years now.