If you've damaged the shock mount it sounds more like you have damaged the front diff case rather than the diff itself. The front diff case has long been discontinued as it was a weak spot on the chassis and are impossible to find.
You can replace it with the Lazer 2000 gear case, which is a replacement but it is made of a weaker plastic that isn't fibre reinforced so you need a Fibre-lyte shock mount that is braced across the top so there isn't the twisting force that rips the shock mount off the diff case. You can still buy Lazer shock mounts from Kyosho but the Fibre-lyte ones are stronger.
The Lazer 2000 diff case is
LZ
4 (B) GEAR BOX FT & RR - L.ALPHA SERIES
and can be ordered through Time Tunnel http://www.tamiyaparts.co.uk/tamiyaparts/tpx/kyosho.htm
or Benracing.http://www.benracing.co.uk/acatalog/...s_Page_21.html
If you need a manual you can download it from here http://www.retromodelisme.com/scan_p...ho-page-1.html