Well I just spoke to Generac and they said that the portable 17500 engine is the exact same engine as the regular home stand by generators shown here:

http://www.guardiangenerators.com/Products/Residential/Guardian/Guardian.aspx

They did mention that it is the same for the 10k, 13k, 16k, 18k units.

The only difference is obviously the fuel source.

They also mentioned that normally the units come with the 50 amp plug, so certain dealers may offer the 70 amp as an upgrade but if I change it out myself, it could void the warranty.

Yuppers...glad I could be of service

The part about not being able to get more than 13kw on natural gas is only because you had a 15kw dual fuel unit. Propane delivers more energy than nat gas, so you get a higher output when using that fuel. The conversion from gasoline to nat gas doesn't harm the generator either. Commercial generators use the same engines for natural gas as they do for gasoline.