Coupled a rebuilt engine to my old Onan generator last year and though quite economical, it was a mistake.
Being out in the Caribbean, I didn't want to rebuild as shipping of parts would have been very costly, especially considering that you may find you need parts you didn't order, on a daily basis.
I purchased a custom rebuilt Kubota engine and mated it up to the generator end, which had been properly serviced. I'm not sure why, but it just doesn't run as well as it did before. Mind you it didn't break down, but it just wouldn't handle more than about a 3kW load on an 8 kW unit, which was tough on the charter cook. We are still trying to figure out why changing governor springs and things but we aren't making much headway.
My main reason for not replacing the old Onan was that I didn't want a solid state controlled generator. I do not trust them at all. Give me a transformer and a few relays I can buy at any auto parts store and I can keep a genset running for over 15k hours!
The second reason was the spare parts. As a charter boat, spare parts are our savior. We had everything from a starter motor to a new, unused seawater pump. No impeller changes in the middle of dinner; just swap out the pumps and it saves 35 to 40 minutes. There are injectors, o-rings, gaskets galore. Suffice to say, plenty of parts, but the freeze plugs and alloy add-ons were going on a fairly regular basis and I thought that a rebuild would be preferable to rebuilding.
So, I do regret not going with a new replacement, though we're still at only about half the cost, with a genset that runs reliably but not at full power.
In my research, considering our generator operates no less than 2 hours a day, 24/7/365, I found Phasor Gensets that appeared, on the surface, to be a pretty good deal, but I could not get any feedback on them through the various forums I asked about them on, which was worrisome.
Panda Fishers are a no go, period; do not fall into that pit.
Westerbeke, Onan and Northern Lights are all about comparable but electronically controlled.
Remember, a sound shield is a difference between night and day noise wise, so if you already have one for your genset, you are a thousand dollars ahead if you purchase a genset that can use the one you have.
Lastly, I ran a sport fishing business in the Bahamas and my Bahamian partner was a close friend of the then prime minister. Any parts we ordered or sent to the US for rebuilding (like starters or pumps) had a 100% duty on them when coming into the Bahamas, even with my political friends, even if they had come from the Bahamas and were used! Things may be different these days, but I would certainly make 100% positive about that before I chose to look around for a mechanic there.
"Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about in boats."
-Wind in the Willows by Kenneth Grahame