Another 3VRH owner here, not in a boat , it's in a 70 year old Ruston Navvy/crane. The engine last had a major overhaul in 1959. Utterly reliable, smokes a lot when it's cold , but always starts, hand or electric.
*My opinion only*: though it sounds and looks great, it is far too big for a narrowboat it will only ever be at idling or just above, the torque available is huge , so biggest prop you can get under the counter is way to go. But , you have it already so live with it. I doubt you will need any spare parts and they will be hard to source anyway- 3VRH hasn't been made since about 1955, when the Ruston Bucyrus 10RB machines went to mostly 3YC engines. You could try asking the Vintage Excavator Trust at Threlkeld if you really feel the need for spares.
Bill