FWIW, I just finished rebuilding our Hurth HBW50. The HBW50 became the ZF 5 when the company was sold. We got the parts from Federal Marine (talk to Jim - he's a veritable encyclopedia of knowledge!). Unlike most rebuilders who use the Alta brand parts, Jim got us original Hurth (now ZF) parts.
Before embarking on this project, we spoke to 4 different rebuilders, and 3 of them tried to sell us a new trans vs rebuilding our old one, stating that the HBW50 is "junk". Consensus seems to be that the Alta parts (which most rebuilders use!) are inferior to the ZF although cost is 50% less! (read: lower cost parts= higher profit for the rebuilder!). I found that rebuilding wasn't that difficult; just need some spare time, a press and some accurate measuring instruments. Complete rebuild manual for HBW / ZF is commonly available as a pdf from various sources online.
BTW, we considered replacement, but ZF5 and 10 are obsolete, and have been replaced by the ZF12 which is approx. 1-1/2" longer. That didn't work in our situation.
(And Jim, the clutch is not bad... typically you only need new friction discs and thrust washers).