hal, I was just talking to a hobby store on this topic today..
If someone has our original Hex Drive Clutch Bell laying around it works with some shims.
The original bells have a 1mm shoulder where shaft meets the bell, 6mm threads and no part number on the bell. 18,19 and 20t gears are available for them. The gears are 13mm wide but should be just fine and they will allow the centre diff to flex back and forth without losing contact with the pinion gear.
We will be working on a bell soon..
We have a few things planned in the next 2 or 3 months
Spools will be coming in 6 weeks or so and we have a vented clutch housing already available...