Choose one of the watch/clock parts to serve as your template as well as the foundation for your brooch.

Once you have something chosen, attach your brooch back to it.

I was lucky in that the holes in my brooch back perfectly aligned with two screw holes in my brooch's foundation which meant that I could just screw the two together using the original hardware from the clock. Depending on the parts you have/find/choose, you may have to drill a couple of custom holes and bolt the brooch back to the foundation piece.