Hurth HBW gearbox

[sv.antea]

I have a question about a Hurth HBW50 gearbox. I have one in the back of Universal M25 diesel that I installed on my boat back in '83. In '85 I managed to fill it with salt water while motoring against stiff norther in the intracoastal. It was still working when I drained and refilled it with ATF few times, but I rebuilt it anyway, just to be sure. Replaced few bearings, half the friction plates and everything was good for the next 20 years. Then it started to refuse to engage in forward. Occasionally first, with regularity later. Checked the shift lever travel, the shifter shaft plate fore/aft position. OK. No, it doesn't exactly slip, which is the problem I read about here often. After moving the lever into fwd and neutral 10 times or so it engages with softer than normal klonk and I can motor for hours with no problem. At first, it may pop out of gear and back in few times at low RPM as if the gearbox needed to warm up. Later on a trip it is more willing to engage again.
Now, I wanted to replace the gearbox many times, but never could come up with the money, so I lived with it. I am thinking about rebuilding it second time now and I am afraid that replacing the friction plates may not do the job. I remember the hubs with the little balls in their curved tracks that generate the thrust for the clutches and I worry that it may be there that the problem lies and my only option would to buy ZF 6M or ZF 10M. Anyone here can advise?