[Dan in Pasadena]

Anything you do is likely to be iffy. That said, I think I'd clean the back side as good as possible and use JB Weld.

A light bead blast followed by a chemical clean would be best but I understand you might not want to spend the money on blasting or worry it would distort the emblem.

In that case I'd mechanically clean it with a wire wheel in a die grinder - obviously being very careful, then chemically clean before using JB Weld. Please post details of what you do.