It is hard to gleem from your post just why you need a back up generator but lets assume it is for mains failure.
Kubota enigines are widly used in gen sets, Janne was correct, that HP will cope with a reasonable size generator without too much frequency drift, say 3 Kw. The bigger you go the larger the frequency drift will be for a given throttle setting, having said that the motor has enough HP to start large loads, assuming the generator can handle it which comes back to generator design.
I would stay with single phase unless you have need for three phase, as I guess you are proposing a back up power supply. With 3 phase generators there is the problem of phase inballance to concider, if all 3 phases are not ballanced then heating within the generator occures.
For standard PTO RPM (yours) you will need to gear up, so my advice is stay with a 3000 RPM generator (50 HZ) or 3600 (60 HZ) and a gear box, as that will be your most cost effective option. If your needs are for AC power then stay with a generator intended for that purpose.
So if a back up power supply is what you need then your Kubota is a viable option, have you thought about a PTO welder, they generally have an AC generator built in and are plug and play. And the welders are around 26V DC so can be forced into action as a charger for a 24V battery bank, albiet with constant vigilance.
A of J