The best plug for Sportwerks motors is a McCoy 59 plug. Medium to hot plug gives better timing to the motor.
You may also want to check a couple of other potential problems.
Tighten down the 4 screws that hold on the engine heat sink. Tighten in X cross pattern.
You may have an air leak in fuel lines. Replace.
You may want to check to make sure your fuel filter is tight in case of air leak.
Your engine backplate may be loose.
Check to see if your high speed needle assembly is tight but be careful to not damage the soft metal
Check to see if your carb body in all the way in.
You could have a broken clutch spring. Take off clutch bell to check this.
See if your clutch bell spins freely. If not could be a bearing problem.
You either have an airleak or something is binding your clutchbell causing a stall.
Good luck.
Rockman