The one that is missing is not too much of a problem.If you engage the carrier lever in it`s hole you can measure the centre of the pin to the teeth on the corresponding gear in the stack.Do it at two or three positions to prove the size.in fact even easier is to measure the centres of the splined shaft and the pin where the missing gear goes,look up what gear will mesh with the stripped one at these centres and Bob`s your mothers brother.Tables for working out gears etc are on HPC Gears website.
If you`re not sure what I mean you can ring me.Contact details are on my website
www.ems-fife.co.uk or PM me for the number.
Mark.