Have had several gearboxes apart including the one currently in the Vitesse in which I had to replace third gear and the cluster, no special tools needed except a length of rod to hold the cluster needles in place. Do not use grease to hold bits in place on reassembly, petroleum jelly, Vasolene, is the stuff to use.
Told some restorers grind down the main shaft tip and fit sleeve or fit oversized input shaft bearing weakening an already weak part, maybe just a tale.
Regards
Paul