Rebuild Onan & Westerbeke Generators or Replace?

I asked earlier quesion about a pair of DD 453's getting repowered or replaced on 48' Hat lrc trawler down in Panama so now I need some opinions on the Generators:
I want to do generators while my main DD 453's are being done. Both Generators have been sitting for a long time and both have problems.
Main is an Onan approx 15kw that is apperantly seized? The back up is a Westerbeke approx 7kw and runs per mechanic but not producing electric.
While Mains are being done should I have these rebuilt and can they be rebuilt in boat?
Do Mains have to be out to get generators out? Would I be better off buying new generators & popping in while motors are being redone? Keep in mind this is down in panama and I have to ship down the motors from the states.
Another question is, if replacing with new genny's should I buy two of the same brand or stick with 1 Onan and 1 Westerbeke to hopefully use same gauges & connections.
If same brand what brand is the best (northern Lights, Onan, Westerbeke, Perkins, etc)
Mechanic states that Onan generator is a discontinued model? does that mean parts not available to rebuild? I can't imagine they don't stock the parts even if boat is from 70's and it's original genny.
Once again I am heading down to look at boat 1st week of June so I will have a much better assessment and model numbers of everything of exactly what's in there.
Sorry for all the questions but everyones responses to my 453 repower or replace was truly very helpful.

That's what I needed to hear. I had a NL on my last boat a 20kw and put over 1000 hrs in a couple of years and never had to do anything but change impeller/oil change and I changed thermostat for heck of it and I had to replace an exhaust elbow and I think I had the heat exchanger tank boiled but I didn't need to, i was just doing my detoits so it seemed like a good thing to do at the time.
Thanks for the impute.

I havn't personally seen the boat or been on it, but I did get a report just saying main gen is Onan and back up is westerbeke. I checked out other 48's and I have been on two and they have had around a 12-15kw main and a back up around half the size. I would prefer to have two different size genny's depending on what I need to run (watermaker, battery charging would not require running a 15kw so smaller genny could be run. When cookies & running laundry and three zones of AC the larger 15kw would be used. I think different size genny's is more of a matter of amount of power to keep genny's under a more useful load which from what I have heard is better for them to be used at 70%-80% load. Carry spares is not a problem in my opinion. But it would be advantageous to keep to the same brand so spares & parts can be bought from the same distributor.