MD6A? that's the little 10hp right? most of those were raw water
cooled if I remember right... so an old one may not last long. Do you have one sitting around or something?. It's not enough for your boat. You need enough HP to punch through chop when trying to go into it... the boat you bought isn't exactly a witch to weather
and having enough HP will be something you want to motor
sail when necessary. (the boat in my avatar is similar and was underpowered... the one thing I really disliked about it 30hp) Why not rebuild
the 4-108? Or rebuilt ones come up on Ebay quite often. Check out Foley Diesel
Foley's website says they have them in stock rebuilt.
There's one on Ebay in FLorida
complete with 2500 hours I think It's about $3500 but may not sell for that as it's hard to verify the actual condition.