The cheapest diesel is USED at your local Big Truck Reefer repair.
Frequently 3 engines can be had , all running take outs for about a grand.
Only one may run $400 or more.
These are water cooled Kubotas or Yannmars and have the huge advantage of very over sized oil pans , to keep them running with no help for a week.
A tiny keel cooler , exhaust wrap and a dry stack would require no marine $toopid priced items to Zero!.
With a hatch in the overhead an engine change would be an afternoon, if that of work.
If installing a diesel I would attempt to locate a gen set noise box set , bottom to top , and install it in that.
If you need a tranny I have a Volvo from an MD 6 18hp that would be fitted with only an adapter plate. $400, plus shipping. Cable shift F N R
Ply boats are OK but an outer covering of GRP or Dynell set in epoxy is worth the extra cost and effort.