Probably a bit late now, but I had a similar problem with my Yam box. Two things to watch out for (hopefully you won't have found out already!) is that the screws holding the box together are likely to break or shear the threads inside befere undoing, and my guess would be a lot of dried grease in there.
Assuming you got it apart OK, bath everything in your favoutrite degreasant, re- lube & problem likely solved. For what it's worth most boxes use the same mechanism, so if it's a total write- off, any controls will do to replace the (mechanical) bits.
You can get a spare set of the plastic bushes for a song or two