[Mart]

The gear might drive ok if slid back to it's natural position. I looked at the cut of the helix and I think it would drive the speedo al right in forward motion. If reversing, though, the gear would pull itself forward and might move far enough forward to lose drive.
You would need to pull the rear end back to allow the missing circlip to be fitted. If you were sneaky you might be able to loop some wire around there and twist the ends so you have a makeshift retainer to avoid pulling the rear back. The correct clip could be fitted at the next convenient time.