Ash,
If you are looking for slightly larger in period they made 14" vewrsions for corvairs (also datsun 240/60Z) you can have the corvair ones re-drilled IIRC.
As for tyre size.. well i went to 14 a few years back for the reason of tyre selection.. now guys are at 15.. and even that is small these days given small toyotas etc run 15"..
If you need tyres for 13" get semi slicks.. falen azenis, dunlop dezeria, yoko RE's etc.. we dont do many kns so you wont wear them out and they provide a good sporting tyre.