That Kubota engine should not be so difficult to get parts for. As it turns out Universal (among others) used Kubota engine blocks to make "marinized" diesel engines for boats for many years. In fact the model number you mentioned "5434" is, I believe a Universal engine model #. You can still buy Kubota parts, usually from a tractor/farm equipment dealer - you just have to know the Kubota engine block # and sometimes even the tractor model it was used in.
In fact there is still a "marinized" Kubota engine being sold for sail boats. The company that does this is called Beta Marine. If you are real nice to them they might be able to help you locate specific engine parts.
As for engine mounts I doubt it really matters much which brand of motor mounts you use as long as they are robust and solid.
I hope you can fix the water intrusion into your cabin on your "leaky teaky".
As BLJones said, get yourself a few rolls of this Butyl tape unless you find it somewhere local (like an RV store): Need Butyl Tape ?? Photo Gallery by Compass Marine at pbase.com
Also check out the projects listed on the website I just linked. Lots of useful tips there.