It's been a while, but I think I remember finding a parts manual for a Duromax generator online that uses the 713cc engine and it had a parts breakdown of the engine.
Here's a video where a guy found and bought the muffler and he shows the part # for the muffler and gaskets.
https://www.youtube.com/watch?v=rAjqJ-cJNZg
But yeah, you'd think that designing, building, and marketing AN ENGINE would be the hard part and that producing a manual with part #'s would be painfully simple and and self-evidentially necessary.
I just don't understand what goes through the "mind" of some of these companies.