I know these are simple mower engines. Running a super hot plug could lead to pre ignition pings. Probably never happen in a mower engine. Not enough load I would guess. I would stick to the stock heat range and deal with the main jet sizing down from 112.
Post up a pic or video of the fouling. Would give us an idea of how much smaller to go on the main.
Make sure she is not passing excess fuel from the needle. Float level the same. Choke plate wide open. Remove air filter for testing. I've had perfectly clean "looking" filters act like the choke was on. New air filter, she ran like a top.