A thumb safety, Is it legal?

Gentlemen I have a question about possible changes made to the safety on an AMD-63 build. It?s been bouncing around in my head for a time, don?t get me wrong I like the look of the AK but I?ll be doing a build using an Ace blank and have been working on a left-hand thumb safety (in my head) for some time. One thing I need to know is if it?s legal to make such a change without getting the ATF after me? I just thought there might be some stupid rule about modifying the safety, believe me I?m into good safeties and would never build something that isn?t 100% as reliable as the original. I?d just like it on the left side, smaller and thumb activated. Or maybe I just want to be different from all the others?