I take it you are planning to heli-coil the stripped threads. This is an expensive process, but it can give good results. In this case, it keeps you from having a problem with the strength of the starter casting around the bolt holes if you used an oversize bolt.
Since there is no M13 standard bolt size, you would have to use M12x1.5. You would not need to use the "special drill" provided in kits, since in this case it is 12.4 mm diameter, and the stripped hole is already 12.7 mm (1/2").
