Hi Jago,
It seems you have a problem with the " PRO-RING". You can machine the pro-ring off the gear and change it by welding on a new one.
I suppose you're in the UK? Try to call Pete@ COGBOX,he's the man who can help you out there.
No need to buy a new mainshaft as the PRO-RING is located on the gear,not the mainshaft
and as long as the gear has no damage you only need a pro-ring.
Best regards Steve