How certain are you it's not ignition issues? If it's low on ignition power, it will foul plugs because it's not getting a complete burn.
I'm no Air Fooler expert, but I've had some air cool two strokes, and liquid cooled. You've stated there were problems to start with, so maybe it's ignition.
Sadly, testing resistance on ignition components won't tell you much unless something is dead. You can use an adjustable spark gap gauge. Those cheap things that have a ground clamp and a threaded electrode. You're able to guestimate spark energy by the gap it'll jump. Drum up some support from the air cooled 250R crowd and see if they have a joule estimate.
Also, on any older two cycle, a engine leakdown test should be at the top of the list when problems arise.
The story of three wheels and a man...