I'm about to buy a used wheel that is off a123 base, will it fit my 126d chassis wheel?

If "fit" means, will it bolt on?, then the answer is yes.

I mean can it be mounted with a tire and used.

Both of the chassis, 123 & 126, were supplied with either pressed steel wheels, cast aluminum wheels ("Bundts"), and in the case of the 123, pressed aluminum wheels. Both types of pressed wheels use the same bolt; the Bundt has its own bolt.

All can be interchanged functionally.

Frank, thanks a million!!