my take---I use Stock, or SPI Hyper Drives.
Every fix kit that I've seen has failed!!
Most fix kits come with forged pistons, great for 4 strokes, BAD for 2 strokes. The forged piston expands at twice the rate of cast. Most snowmobilers are too dumb to run forged! See it ALL the time, cold seize!! Every fix kit failure I've seen is a cold seize failure!
If you are smart enough, and have the right clearances you can achieve success with forged pistons. I've ran them over the years with success, BUT you need to warm the sled up, take it easy the first 3 or 4 miles, and get the the