If it's properly maintained, there is no reason to not expect 50+ thousand hours on that genset. We have larger gensets on the tugboat I work on, and we don't even think about overhauls until they have 30,000 hours...and expect to get in excess of 100K before any talk of replacement.
Granted, these are larger commercial units, but if you maintain and follow the recommendations for the engine and electrical end, there really is no reason you shouldn't get extended life from yours too.
I agree, if you don't know enough of the history of this particular engine and generator, you may want to have it "serviced" by Onan, to see if it's running up to their specifications....and if there are any things that they might recommend being done with it. (such as belt change mentioned above)
Good luck.