For starters, it would be a power thief on a low horsepower bike, as the belt has to have a fair amount of tension to prevent the belt cogs from skipping on the pulleys. Another issue is that to maintain that tension consistently throughout the rear suspension travel, the drive pulley would have to rotate on the same axis as the swing arm pivot. If someone did make such a kit, it wouldn't be cheap either. Conventional drive chain and sprockets are still the most efficient means of transmitting power to the rear wheel on a small displacement motorcycle.