[Faster]

Beta Marine makes 'drop in' replacements in a diesel.. but not cheap (no repower is inexpensive) Going from gas to diesel also has its complications in cleaning out the old tank, plumbing for a return line, etc.. all do-able but very expensive esp if you're paying someone to do it.

I think you need to set a budget and see what can be done within that.. the DIY involved the more you'll save.