I have had nothing but problems with parts unavailability on every Onan generator I've had. That company is the absolute worst about discontinuing parts support when there are still thousands of that model in the field to try and make you buy a new one. I would never buy another Oh-No. If I was buying a boat with one in it that was more than about 5-10 years old, then I would factor replacing it into my offer for the boat. Their policy about cutting off parts support much earlier than other manufacturers is really that bad.
I have personal experience with Phasor and Northern Lights, and highly recommend both. Phasor is unique in that there are no proprietary parts anywhere on the whole thing, everything is an off-the-shelf component that will be available until kingdom come. The engine and engine control systems are a mass-produced kubota, the electrical side is all mass-produced off-the shelf stuff, you are never going to have parts issues with a Phasor. You won't with Northern Lights either, because they don't cut off parts support on you like Oh-No does.
I could not urge you strongly enough to consider another brand besides "Oh-No I can't get parts!"