Backups are specific to the router you made them on due to things like serial numbers and MAC addresses. You can’t use a backup from one 433 on another let along another model.
Sadly applying an export script over the top of a default config also has issues, since very often it will fail trying to add things that already exist.
Your best bet is probably to reset the rb750g to the default config, export that and compare it to the original export with something like meld or kdiff or … Then you can go through and apply the changes needed to make them match up. I discover this after accidentally locking myself out of the router (a botched cut & paste) and having to reset to the defaults.
If you’re any good at scripting it would actually be fairly straight forward to convert an export script into one that can be applied on top of the default configuration. Mostly it’s a matter of adding “remove” statements before the “add” blocks or converting them to “set” statements. For example one of the first conflicts I ran into was with this command:
/ip pool add name=default-dhcp ranges=192.168.1.100-192.168.1.254
Converting that into a set is fairly simple:
/ip pool set [find name=default-dhcp] ranges=192.168.1.100-192.168.1.254
Going through the whole export file like this would be fairly tedious, but when you’re done you’ll be able to restore the configuration any time you want.