Anyone know of a source of the special clips that hold it to the grill? Mine fell off the car sometime while driving and I have been unsuccessful finding an alternate way to secure it.

The repro emblems come with the stamped steel quicknuts that cut threads in the shaft. They seem to work reasonably well, if you are careful not to break the studs.

I didn't trust that so I put a dab of silicone on the posts then inserted them into the grille. After curing, they will not fall off, quicknuts or not. This should work on the stock emblems as well.