As I said above, you don't know how it's wired up.
There could be two switches, one for forward and one for reverse or it could be one DPDT switch which forms an h-bridge in itself.
If it's two switches, then you'll need a 555 for each and three buttons, one for forward, one for reverse and one for stop (reset). You could use the existing rocker switch for the forward and backward and add an extra stop switch.
EDIT:
Please post a picture of the connections on the back of the existing switch.