Original locks can be rekeyed, but I know at least in the US, there are very few locksmiths that still stock the tumblers. If you're lucky you could maybe find an old school locksmith, or if you're very lucky you could find an old kit. If not, you can always file your own tumblers out of some brass sheet. Here's a write-up on the process of rekeying Datsun locks.
https://ratsun.net/topic/47412-how-to-dissasemble-datsun-510-ignition-switch-and-rekey/