I don't know how big your wanting, but the 20K onan is a 4 pole generator (1800 rpm) . The 20K Generac is a 2 pole (3600rpm).....but I expect you'll pay a bit extra for the onan.

Briggs/straton bought generac a while ago......so I would assume the build their own engine.

Google both onan and generac.......their specs are there.

Have a good night,

John