[Cuog]

As said above my garden tractor actually uses a belt as the clutch and it works very well in the 10 years we've had it the belt has never brokem or popped off, and this one has a 21 HP engine with lots o torque(for a small briggs anyway)

I would use 2 pneumatic pistons driving an "idler" sproket onto the pulley you want operable. what you may want to do it have pots on the oi that controll how much pressure is applied to the cylinders and the pulley so that there is enough tension but you dont snap belts,

also with 2 pistons you can cntrol them individually so that one is not dependent upon the other in case a belt does come off you just stay in the other gear.

To keep the belt from jumping off the track, make the track too tall for this to happen by adding a plate that is about and inch larger than the pulley and if the belt pops off then you have other issues

Also something that ensures the belt stays in its track like pins that it will rub against and keep it inline for the match untill the misalignment can be addressed

I have seen all of these methods used successfully in various applications with the exception of the dual cylinders that is merely my own creation