Andre, unless your starter has been severely overheated (or on fire), or has had a structural failure, almost any starter can be rebuilt to better than new status. I would investigate all the commercial marine starter rebuilders and beat some bushes.  Rewinding and applying resin to armatures has been very common for decades.  Cast iron can be welded by talented craftsman and this is done in the antique automobile and vintage race car industry frequently where replacements are not available.

I know of stationary 4-236 engines to go 25,000 hours when used in agricultural irrigation applications. Once they are turned on they are rarely turned off. They have dual/separated crankcases and filter media so that new oil can be introduced without shutting them down. When they reach 25,000 hours, the guys who maintained them would purchase them, put new rings, bearings and seals in them and use them for their own projects or sell them to anxious waiting buyers. I saw one in process of an overhaul and the pistons were removed from the top of the block, not dropped out of the bottom. NO ridge at the top of the stroke as is usually present with this amount of hours.  Incredible. This was due to no heat cycles/temperature fluctuations, clean lubricants and excellent/consistent metallurgy.

The negative to this plan is other ancillary equipment such as heat exchangers, pumps and other essentials will become rare, then unobtainable. Even now new injectors are harder to find than a few

years ago. And since this is a dirty rotten stinky analog dinosaur, it could be legislated out of existence.

Seriously, when importing a used boat into the United States and some European countries, you must certify, under severe perjury penalties , that the engine in the boat is original and meets the tier emission standards in effect at the time , OR, if a replacement engine, it too meets the same standards. If the engine is a replacement, be sure to get manufacturers supplied proof of compliance with the appropriate regulations. I learned about this from hard experience with customs officials who don’t really care about anything than their retirement package.

If you do replace, Beta was a good choice but haven’t had any exposure to them for several years. My go to is Yanmar and I am proud that I pushed Amel hard to switch to them when Volvo started to get hinky with warranty reimbursements. Big leap of faith for Amel as Yanmar was not as well known in Europe and was still developing a sufficient dealer network. It all worked out well.