If the heat sink doesn't do a good enough job of holding the transistor in place, you could look at using an epoxy such as JB Weld:
http://www.jaycar.com.au/Service-Aids/Chemical-Aids/Adhesives/J-B-Weld-Epoxy/p/NA1518to fill in the burnt out area. You'd need to put something underneath the board to hold the epoxy in place as it sets - I've found that most clear plastics from product packaging works well for this - just tape it in place.
Then drill some new holes for the transistor and jumper the legs to the relevant tracks as suggested by Steevsee.
