Easiest if you have the ignition switch out for easy access....

Find the (probably big red wire) connection that is always hot (check with volt meter, easy enough), turn ignition on, and with a jumper wire, one end on that hot terminal, go around to the other terminals on the switch until the starter kicks (I'd pull the plug on coil before doing so, so it doesn't start unexpectedly).

Once you find the one that activates the starter, well then you're done.. Just wire your push button starter to those two terminals, leave everything else in place.

That way, your key will still work like normal, should your starter button fail or you want the originality option.

--
-Matt I ♥ my ♂