For what it's worth my 1956 SIlver was re-engined in the 70's ( I think maybe as a result of sinking but not sure - lots of port side planking had been renewed) with a couple of marinised Ford lorry engines. coupled to Velvet Drives They're referred to as 2701's - 4 cylinder models and I think are rated at about 80hp. They were marinised using standard Bowman heat exchangers etc. One benefit is that I can go to the local Ford commercial dealer and get a set of gaskets etc. The other alternative for a slow revving engine would be an old Perkins - 4107/8. If you watch Travels with Geordie on UTube you will be aware of his involvement with various types of Perkins but lastly Beta. It all depends on your pocket I suspect !!