2002 800 rmk running on one cylinder

the wiseco kit worked great for me.
make sure your water pump belt and pullies look good. and primary clutch.
then go for it.

Most likely an electrical problem. For instance, the spark plug may be fowled or the spark plug cap may be loose or shot. Take a look at the plugs, and replace with new ones. Remove the exhaust header and take a peek up inside the cylinder exhaust port to inspect the piston sides for damage. Look inside the spark plug hole for piston burning or melting. It is possible that the intake reed is missing one petal, then one side won't run very well or not at all. Inspect the VES exhaust rubber gasket real closely and replace the paper gasket between the VES assembly and the cylinder head. Clean up the VES guillotine valve as it may be frozen up with oil.

Very common for the coil to go on those...witnessed it more than once for that era. Check the obvious stuff first (plug spark, plug condition, caps, wire, etc.).

A fouled plug did it to me once and scared the heck out of me cuz I thought I burned a cylinder down.