I suspect it is a Farymann A30 - the same
engine that I have.
I know of no reliable source of parts. I've stockpiled a few things over the years (gaskets, piston rings, injectors, shaft-couplings, etc.). Some items common to other engines, such as injectors, starter, alternators, etc. can generally be found.
But I've had trouble finding anything particular to this
engine (pistons, piston liner,
water jacket,
head, etc.) short of locating a donor engine. The only other idea is to keep an eye on ebay.de (Germany) as sometimes parts can show up, but it can be a trick to get it shipped stateside.
I'm not aware of frost/freeze plugs on the A30. I believe I've read an account where someone successfully repaired/replaced the water jacket with PVC. From what I
recall, it's plastic and could be repaired with fiberglass/expoy, potentially. Regardless, it's not often the water jacket that fails... it's usually the grooves in the cast-iron cylinder that seat o-rings that go. Short of locating a new piston liner (I've never seen one), I'm not sure there's a good fix.
A story - we sheared our shaft coupling heading into Sturgeon Bay, WI last summer. I couldn't find a replacement and started trouble-shooting some modern solutions that were tenable, at best. Eventually, I located a used part in some guy's basement in
Germany. A few weeks later, installed and on my merry way. Subsequently, I was able to find another NOS part and order it (so now I have a spare).
I mention this, as I think it's sort of the common approach with repairing these older engines. In my experience, they're pretty reliable, but when something does go wrong, it's a headache to locate a solution. Long-term, my
boat (Tartan 27) is probably a candidate for an
electric motor or a rebuilt
diesel.