Eventually you will find another 7s Alpha 13-28t freewheel, but in the mean time, I believe you can ride for some years on the 13t cog having a broken-off tooth.

I would normally source such a truly excellent freewheel from either a discarded bike or from a bike co-op type of place, usually found in a good-sized city.

Discarded bent wheels are also often found near urban encampments, and if there are "residents" there, a few dollars might get you the exact part that you need.
No joke, I have sourced valuable parts this way. Oh, and don't forget to visit any bike swap meets in your area. N+1 Baby!