Predator 212/420 durability

Splash lubed engines aren't lubed properly at reduced RPMs; running it slow will kill it quickly. I realize it's way too late now but I would have gotten the 212cc and run it at full speed, they're way quiet compared to a 1985 Briggs or Tecumseh and a simple sheet of plywood between you and the generator will reflect the sound away from you and make it inaudible from less than 50' away.

Don't get a K&N. The utter lack of filtering will kill the engine long before you recoup the initial cost of the K&N in "disposable filter savings". Take the paper filter out and knock it on the ground a few times to shake the loose dirt off when you do an oil change (don't use compressed air because it can cause little tears making the filter as worthless as a K&N). When you hold the filter up to the sun and can no longer see light coming through the filter (after knocking the loose dirt off on the ground) then replace the filter. People get all worked up over 1973 energy-crisis fake-news about air filter elements being super restrictive... Check out this article from a real garage-scientist: I can't find the original article that featured that Toyota air filter but if I recall correctly that black filter had over 230,000km (124,000mi) on it and when knocked on the table a pile of loose dirt fell out...again from memory but I believe that filter had 0-1" of water flow restriction (basically none).
What RPM and (estimated) load did you wind up running at?

It's my understanding that running a petrol engine off propane loses 20% of hp and then another 20% is lost for NG.

All this to say, the bigger engine was the better choice this time around.
Be sure what ever you do the generator spins at least 3000 rpm for proper cooling, 5 K is preferred.

The engine must spin at least 3000 RPM for proper cooling air flow. at high loads.

Best is use 2 to one for generator, motor at 3000 RM, gen at 6000