Just to be clear...im not using the 3910X for any reason other than that is what the original builder put in the engine when he built it, so that is what I used. Since then I have added a good bit more compression with a domed piston, and a bigger carb using a 28mm Clements pumper carb instead of a 304 dominator Tilly. Some of the tracks we race at will completely stop the field during every caution, those are the tracks where the plug will foul usually. I have no problem fouling the plug on other tracks that allow us to keep moving under caution and never completely stop the field unless there is a bad wreck, which then we usually shut off the engine when we stop.Flattop gave a variety of good choices. I really like the Denso plugs we also stock and sell them, although you may or may not find them at your local Walmart or auto parts store - I have no idea if they stock them. The problem with the fine wire plugs is that they foul even easier. If you find that you're fouling plugs...my thinking is that your problem is not with the spark plug heat range, it's with your carb tune (or you're simply letting it idle on the grid too long.) This has always been a problem with bigger Tilly's drilled with a very rich pattern for low rpm and a reason why I like using the Mikuni accelerator pump carbs instead.
My experience has always shown me that the higher the compression, the colder the plug that needs to be used. It would seem to me that you're wanting a hotter plug thinking that it will better burn a too rich condition that is fouling your current plug. A hotter coil, with more voltage at the plug gap, would make more sense to me.
The 3910X is a fine plug for a stock class application where you are simply trying to displace the volume in the combustion chamber. Don't believe the hype of having several (more than one) ground straps. I do, however, like a bigger flame kernel (which also helps resist fouling.) It'll take a big charge to jump a larger gap.
Thanks and God bless,
Carlson Racing Engines
28 years of service to the karting industry