I may have an extra in my box of parts...I too have a '79, and I recall I got a used one a few years ago when mine stopped working...however, I don't think I ever installed it, as I learned you can get into them an refurbish them pretty easily...which is what I ended up doing. For mine, the seal had failed and the starter filled up with oil. Once I got it all cleaned out, put on a new seal, it worked great. Not sure if yours is fixable...but worth a try.
As to my spare one...I'll need to check my shed, but I won't be home until after this next weekend...let me know if interested and I'll see if I can find it.