Remember the stop hotkey doesn't merely stop, but is also set to back step. While I have only used the hot keys (I started using ExpressScribe ca. 2000, or earlier), many if not most individuals, especially professionals, are using foot pedals. There are other programs with a play/pause/backstep/play toggle, but they have far, far fewer compatibilities with other audio formats than Scribe. And they cost.
I can suggest a workaround, which will not be exactly a toggle but use the same key, which is to change a particular function key, let's say F9 for argument's sake, to play, and Ctrl F9, or some other combination of keys with F9, as your stop. I tried doing that, but I was too used to the F9/F4 combo to change to a different pattern. You may have better luck.