The centre bolt should come out OK. Keep winding! It's really just a question of getting the steel collar off the inner side to get the main spindle out (it's on quite tight usually, or should be), then unriveting the pinion from the metal sandwich it's in, and installing the lot on the new gear, re-riveting, pressing collar back on, etc. Also worth getting new springs for the weights. Assuming all bits are serviceable, they're not so bad to muck about with. Following the death of my fibre one, I installed the ATD unit in an alloy gear. But worth looking closely at all parts for slop, wear, straightness, loose pivot posts for the weights, tired self-extract thread etc. How that works is easier to figure by looking at it than to describe - but it's obvious when you have one on the bench.
Because in the meantime I'd put a manual mag on my A (with a plain alloy pinion), the modified ATD is still in the bits box waiting for the next time - and for some new springs for the weights.
I think the alloy gear is marginally noisier than the fibre one was, but otherwise I reckon it has to be a better option. Think I said somewhere before that I know of 4 stripped fibre pinions just in the last year or two, on 4 different marques - and it's a PITA breakdown to suffer. It was of course snowing when mine stripped. There's no reason not to use a metal pinion AFAIK - my AMC twins use steel.
Good luck!
