Sorry to hear about your transmission troubles. We also had the Hurth 50 transmission on our 1984 E30+, and it slipped periodically in our initial years with our boat, making a real gear grinding noise. When I put it in neutral, and then gently increased throttle, it would be fine until the next time it slipped, which could be an hour, or a week or a month later. I couldn't live with this uncertainty, so decided to hire a local mechanic with a good reputation to replace it. All the info on this site and others suggested that rebuilds are "iffy" at best.
Our mechanic recommended the Twin Disc MG340, and it fit as a replacement with no modifications necessary. It worked fine initially, but then also slipped a few times, though always months apart--ie. once or twice per season. I changed which transmission fluid I use, and also increased the frequency of transmission fluid changes, so that I now change it whenever I change engine oil--about every 75 engine hours. It now hasn't slipped in years and seems to be operating fine with no issues at all. We have about 1,500 hours on the new transmission, the Hurth was replaced at about 800 hours, so so far I'm way ahead with this transmission.
The twin disc has a good reputation. I don't know if there is a better one out there, nor do I know if it will be a direct replacement fit for your boat, though it was for ours.
Good luck!
Frank
