Belt drives are as efficient as a PSI motor.
He's right. Just because the one that you saw w/ a belt drive ran better than the one that didn't doesn't mean anything. I've seen two identical stock sleds, identical clutching, jetting, mileage of 100 miles, suspension set up, gearing, and riders within 10 lbs of each other. One consistently, not just beat but KILLED, the other sled. Any experienced rider knows that many a tuned stocker sleds have beat many a mod sleds and it will happen time and time again.
For the cost of around $2500 you could get a lot more performance for your money. I can imagine Mark laughed at you. I would've too. Why would he want to build one??? Not enough demand to warrant the costs of design and production. You'll get more out of your money if you just pay a few snowesters to teach you a thing or two about the sport we call snowmobiling. From your recent posts it's evident this could help.