International layout Model M have been manufactured mostly in the "classic" version (1391404). Sometimes you can spot one on ebay.it.
Once you have one (or more
), you can look for a Unicomp Model M 85key spacesaver (UNI04C6), which is the only M spacesaver with ISO layout ever produced, and swap the keycaps with the 1391404 as you can see
here.
Regarding the M13, I've seen some with ISO layout (IIRC french or canadian layout), if you can get one you might be able to do the same keycaps-swap with the 1391404.