When you put in the gear cluster, perhaps you didn't connect with one of the selectors.
Whatever you have done, it has to come back apart.
The only saving grace is that you don't need to touch the top end.
After you remove the lower crank case, check that both gear clusters are hooked up with their respective selectors.
If they are you should be able to select the gears by turning the end of the selector drum while turning either of the gear shafts.
Sam.
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