I believe that replacing the generator in its entirety would be better. This generator is still good as a running take out to someone and I have tons of spare parts to go along with it. The choices are what have me analyzing all models and I like the idea that the NL and Onan's truly have a single service side. Others I have seen have the raw water impeller buried low and on the opposite side.
I have the NL 9 kw unit.
Everything is on the service side, and the seawater pump is on the front, so you need two free sides to service the NL units.
As far as replacing the generator end goes, depending on the generator that can be a cost effective approach.
The smaller generator ends, if they are utilizing SAE sized bell housings can be replaced with often better quality generator ends for <$2K in parts.
The Onan MDJ series of generator ends have no new replacments available. They utilized the YD generator end which was a great end, but it is taper fit on the crankshaft, so again no new replacements but plenty of NOS parts around.
The problem with small generator major repairs is that if you are not in a position to DIY the job for any reason (time or skills) then labor can quickly turn a generator repair into a endevor that would cost justify a new generator.
Here's a good example. As part of my living I work on generators. Been doing it for decades. I bought a boat and for time reasons I needed to hire out some generator repairs. (the boat was in Anacortes and I am in Alaska)
This was a westerbeke generator. The generator end was needing replacement, and the heat exchanger was needing service. The shop called me and at their labor rate of $90 an hour they estimated $4800 in repairs.
For me it was an easy decision to opt for a brand new NL generator. The total cost of that, installed was much more at $15K but there was no way I was going to drop 1/3 of a new unit price into the old westerbeke, which in my opinion isn't the best of designs anyway.
Now, if my boat hadn't been 1500 NM away I would have yarded out the old generator and refurbished it with a new Marathon generator end, digital control system, etc... My cost then would have been a little over $2500 but my labor cost would have been "free"