I really have too much time on my hands!
I've been researching things on Kubota engines, transmissions, water manifolds etc.
Basically it seems if you DIY the marine conversion of an engine.. say out of a tractor or from a vendor.
Marine manifold (some have Heat Ex built in)
Raw water pump. (and a way to drive it)
Panel and gauges,
Wire harness ( or make one)
Bell housing and adapter for the transmission.
Water lift muffler and gooseneck
Seal or stuffing box.
flywheel and plate.
various hoses, tubes, connectors, adaptors,
Electric fuel lift pump (or mechanical)
Raw water strainer (should be large)
Add "sweat equity"
All this above and more.. is it worth it to save $2-4,000?
I'm not going to do this, or ever attempt it, but It does seem to be more popular in other countries.