Just suggestions of what I would try:
Are the spark plug caps still the metal style that SkiDoo used for years? They have proven troublesome after many years....replacing with some new NGK caps has been a common practice for lots of us SkiDoo owners when encountering spark issues....plus you would trim the spark plug cables back 1/4" to get to fresh wire in the cable.
Proper spark plug, with proper heat range, CHECK the gap, etc. ?
Is it premix or injected? Using decent oil and injection set correctly?.....might try one heat range hotter plug.
Probably running rich fuel/air mix....were outside air temps rather warm, or does it foul plugs on very cold days also?
Probably jetted rich from the factory..??....try dropping the jet needle down one position....unless spark plugs look too light in color.
How do plugs look when removed?? Really dark? always the same side?
Fresh gas in the tank? What are the chances there is some water in the gas tank?
Does it have an enrichener (choke) on the carb? If so, that plunger has to fully seat in the carb body or it will run rich.
Observe primer tubing.....can you see fuel being drawn through primer when engine is running?....if so, time for a new primer.