Generally the sliding and cluster gear should be replaced as a pair if possible, the drive gear not usually that worn, as it is engaged constantly with the cluster, and if the dogs that engage with the sliding gear are unworn , should work fine with the new gear.
Unless .... ask Juergen if the dogs on new gears are undercut at all . Sometime in the early forties on 741 and sport scout gear boxes, Indian started to undercut the dogs slightly so the sliding gear and drive gear had a slight dovetail engagement, which lead to far less kicking/slipping out of gear. I beleive Jim Parker in Australia does this to early scout transmissions, don't know the details, never done it myself. Would be interesting to hear from anyone who has.
