Bernina 730 record - how easy to take apart to replace a gear?

I have a beautiful Bernina 730 Record which has a badly cracked gear.
I don't know the name for it - it sits at the top of a vertical shaft near to the handwheel end. I think it may be the hook drive gear but not sure.
does any one know where I could get a replacement and could you also tell me how to fit it.
I realise this is a big job as it seems quite a lot of the machine will be dismantled.
A repair shop has quoted £150! No way can I affford this and it seems a great shame to just let the machine go for parts as the rest is great.
Thank you for any help at all here.