This boat was originally powered by a Volvo 15hp inboard diesel. The P.O. replaced that with a Yanmar 1GM10. What prop and Beta engine do you advise? I think the shaft might be metric. Is that an issue?
The Beta 14 will be a great engine in your boat. It is certainly underpowered with the 1GM. If you use a 3 blade prop, a 13"x7" RH will do fine. If you still have a 25mm shaft, we can supply the correct shaft coupling so that is not an issue.