We are a long way away from knowing if your engine has minor or major issues.
Low power and black plugs is carbon fouling. One cause of this can be a plugged or collapsing exhaust system, which will inflict the same issue on any replacement engine. Also step 1 is make sure the bottom and PROP are clean. Our props are small and it takes only a few barnacles to hose up the system.
What is your budget like? If $15,000 is pocket change, a few phone calls and a brand new diesel is a good idea. If that is not the case, we can help.
Joe
Coquina
C&C 35 MK I