Hi, trying to help out a customer with a MC21 (dry clutch model?)

He is using it on the track in Bermuda and keeps lunching gearboxes.

Am i right in thinking there is a "kit" gearbox available for these and if so does anyone know:

1) Where to get one!
2) What ratio's they are
3) Are they more hard wearing than standard items (undercut dogs etc?)

Cheers,

Sam