Given the amount of conductive gold remaining, the rear diaphragm may still function (you did not mention whether it does).
But the rear may have suffered too much loss of s/n, due to loss of gold and/or heavy contamination.
Though the front side looks also moderately contaminated, the mic should work fine in cardioid, as long as the membrane tension of the rear is still ok (it should be unaffected by the gold loss).
Your options, then:
1. Leave the capsule as is and use the mic, at least in cardioid.
2. Find a used, period-correct (second generation) K67 and either replace the whole capsule, or, if you are really happy with the mic in cardioid, sandwich the current frontside with one half of the donor capsule.
3. Replace with a newer K67. If you can find a K67 made before 2000, the overall timbre of the mic should be relatively similar to that of your original capsule. (You had mentioned you thought that new K67/870 are scarce. That is not the case: you can buy a new one from Neumann without any wait, but that version has an audibly different timbre.)
What is not an option: to assemble a non-brass K67 half on the rear of your current brass K67. I have not been able to mak ethat combo work in the past, due to various design differences.
Let us know what you decided to do, and how it affected the sound.
