ATC would most likely have you rejoin the SID via a waypoint. So you just push Direct and the waypoint and go back to NAV mode. Otherwise you could just arm NAV and fly an intercept heading until you are back on track.
As for heading vs track, I can't speak for all Boeing and Airbus products but for A310/320/330/340 you can fly a track between 2 points, or a heading, but you can't just dial in a track the way you do a heading.