You can specify a range of ports to be used, e.g. 12300-12399, and it can select one from that.
This is configured in IP->Firewall->NAT.
You will already find a rule there. Copy that and set the UDP protocol and dest. port 123, and change it to src-nat with the proper port.
You will also need to insert the external IP address in there, hopefully it is static.
(with masquerade instead of src-nat you don’t need to specify that, but it cannot do the port translation)
After adding the new rule, move it above the existing one.