Have run into this problem with cars and bikes both. Try this. Get some silicone spray like 3M pure 100% silicone or similar. Spray it on a rag good and wet, then wipe and massage it well into the full length of the plug wire, including plug cap and also where it joins the coil. Wouldn't hurt to treat the wires leading to the coil if you can reach them too. Heck, just treat any wires that look like they might get wet.
I've used WD 40 for this in the past and it works too, but I don't know how good that is for the insulation in the long run. And it'll surely put an ugly etching on plastic parts so be careful if you use it.
Plug wires and other wires can have micro cracking in the insulation which let moisture in. The silicone will seal these cracks up if they're not too bad. Might have to do it once a month or so but if you don't have the bucks to spend on new wire, coil, cap, this method might get you by just fine.
If it's moisture getting in the stator, you've got to seal that better somehow. Perhaps a tube of silicone gasket ?? Don't know for sure. I had a "72 Kawasaki 350 triple that would die every time I hit water or rain. Or even a himid day. Sealing the points cover with silicon gasket maker finally cured it.
