Thinking about it further the gear needs to be all the way back so it keeps the roller bearing located properly in the torque tube. It might be best to bite the bullet and pull the rear back and fit a good fresh circlip.
I would try using a short piece of copper wire 12 or 10 AWG NMB, i.e., household wire. You want the wire to stand proud of the slot. 12 AWG would be easier to twist the ends of and then bend the cut ends up toward the universal joint. Use a drift pin to help bend the twisted ends forward. Looks to me that there is enough space to do it and sure easier than pulling the rear-end back.