To all posters here -
Contact Marine Diesel Parts Warehouse
I run the MD7a in my Islander Bahama 30 - Original engine, and no idea of original (before me) hours - I have replaced several parts (Including Inj pump) and have found these guys to be the best prices and service by far - they have a great stock of new parts as well as several spare engines/short blocks/parts only units on their floor - they are also very technically adept and know the engine inside and out.
These engines are typical small diesel engines - all it really takes to make them hum is compression and fuel - if they are not running, this is usually the case, and more than 50% of the time its a fuel issue -
Before re-powering, or throwing out the baby with the bath water, give these guys a spin - I'd be surprised if your issues were not solved.
By way of full disclosure, I am NOT connected to this business in any way, but if you've read any of my other posts, you know I do not beat vendors up, but rather share the great ones for everyone's benefit - Fair Winds to all.....