You can pick up all the parts(shafts,gears,casings,motor plate,etc)for the
Ansmann Macnum/Mad Rat 'box for £16.99 on E-Bay.
http://www.ebay.co.uk/itm/ANSMANN-MA...item483fd9e9da
http://www.ebay.co.uk/itm/ANSMANN-MA...item48412cbd6f
http://www.ebay.co.uk/itm/ANSMANN-MA...item5d2e30d518
That doesn't include bearings(15x10x4,2 off/10x5x4,4 off) though.
You'll also have to make up mountings to suit the Cobra chassis and
rear shock tower.
BTW,Macnum g/box parts are the same as the 'Rat,but a £1 cheaper.
Search Mad Rat and you'll see what I mean.