G'day Tom.
I've only ever used NGK's in everything (BSA's, Honda's, HD's, mower, chainsaw etc).
If a plug is fouling try a hotter plug, say go to a B6ES/HS or even a B5ES/HS or a BP6ES/HS (protracted electrode). ES for alloy head, HS for iron head. 7 is standard but I run BP6ES in the 51 A7 and BP8 or 9ES in the A7 racer.
Don't buy plugs off Fleabay only from a reputable supplier.
Cheers