What does the fouled plug look like?
Is it clean but wet with gas if pulled immediately after running? White vapor smoke, eye burning, raw gas smell.
That can indicate weak or no spark, low compression.
Is it flat black soot? And the smoke, black smoke?
If so, those are indications of too rich.
Has the air filter been serviced and checked for restriction, mouse nest, mud dobbers, etc? Will it run better disconnected?
Any flooding, will the carb adjust? Choke fully open?
Or does the plug fouling have a glossy black, or black crusty appearance, the electrode bridged with carbon?
That indicates oil burning. Could be rings were damaged on install, problem with the bore, cylinder wall, ring land damaged or broken. Sometimes rings will be slow to seat, but that is usually not severe enough to foul plugs. Blue smoke under load indicates rings, on start up indicates valve guides.
A compression test might give a clue, also excess puffing out the crank case breather tube indicates blow-by, leaking rings.
Plug Chart