If I had a nickel for every "mechanic" that jumped into mopeds with a high-dollar build only to be left angry and cashing out in one year, I would probably have a few dollars.
Your advice was incredibly wise. If he's not willing to listen to you, I don't think he'll be keen on strangers advice.
That said: shallow case matching to keep case volume low/pumping pressure high; don't go crazy with carb size (consider 24mm the maximum); don't skimp on the crank, ignition, and clutch; gasket maker does not replace a paper gasket.
GL and Godspeed, Gila Boi