MTU 8v183

Yes, but the overhaul is done in place. That is required by MTU given the age, if you look at the owners manual, and they're close enough on rebuild (via hours) that you'd already be spending 1/3 to fix the issue you have now and 6 months later Boom. The fact there's a new short block on one side, makes me worrysome of the other side. Much cheaper to rebuild in place, versus replace the entire short block due to a catastrophic failure and having to get the short block out and in..........ALSO, very important always change both belts AND make sure they are always tight and good. IF you have a belt failure on a MTU 183 series, the motor overheats and grenades before the alarm even sounds. Also the belt tensioner swing arm (adjustment arm) likes to freeze and it's a $5000 fix, so work it every 3-4 months.

New member name?
Usually a flywheel pickup sends the tach signal.

I think 650rpm in gear. Governor adjustment somewhere but I could not tell you exactly how to do it.
This would be one of those explore and conquer things for me.

What kind of synchronizer do you have.
If an ole Glen-dinning, Follow the mechanical tach cables to the box. Mag pick-ups could be there.
Still think a flywheel tach pick up.

That's only 30 hours per year, not a lot of use. I'd have it surveyed by a good MTU outfit.

Could that be 900 hours since overhaul? Mistype and really 9000 hours?
At 30 hours a year, boat must really be a dock queen.
I would be worried.
Please have the boat and engine very well surveyed.

Define "normal", also because while 900 hours is not a lot, some maintenance jobs are either hours or AGE dependant.
There's way more than oil and filters in 30 years of engine life, regardless of hours!
Just check out the official MTU schedule for the TE93 variant (TE92 is practically identical, in terms of maintenance), but take a sit before going through it.

The good news is that there is actually one big difference between the TE92 and TE93: the former was the very last among their 183 series Mercedes-Benz engines to be 100% mechanically driven, while the latter was the first (and last!) electronically controlled, before they phased out the 183 series altogether.
What this means for you is that you do NOT necessarily need an MTU specialist, which at least in Europe (and based on your location I guess is your area of interest) have a bit of a reputation for being outrageously expensive.
In fact, any good mechanic with some experience should be able to handle them.
Particularly MAN dealers, because their engines of the same vintage were almost identical, aside from some marinization differences - but again, with no electronic complications whatsoever.

What boat R U buying, if I may ask?
I'm not pretending to have seen each and every EU model powered by those puppies, but I definitely came across a fair share of them!

Nice solid boat, and larger than the name suggests!
Besides, Ferretti build quality back in those days was comparably even better than in much more recent production.
I love for instance the two additional side hatches in the cockpit, for better access to the engine room.
Actually, it was a layout that they borrowed from Mochi, but God only knows why they abandoned it in all following models!