massimos are known to have overheating issues.. look at the recovery bottle when its running and hot.. if you see tiny air bubbles, you have a blown head gasket.
if its just a rich fuel problem, and fouling plug in 5 minutes, you should see black smoke and the exhaust should burn your eyes. Did you try just changing the plug or wire ? Is it dry black carbon fouled or wet fouled ?
are you sure its not water? Dont have any issues with my Turbo Diesel 40HP tractor in cold weather with nothing but Lucas in it. ... .we got in the teens here too for 2 days. started right up.
problem is the air blocks the water passage on top of the head.. you gotta bleed it there and mine has bleed at waterpump. some air was trapped there too.. If air is in it, water will not circulate and it will overheat fast.
my Hisun in my Massimo , sold by Tractor Supply, assembled in Dallas, say Made In China on every part of all of it .. Altho it now has multiple Yamaha parts mixed in. lol