Here is what happened to the GasGas gearbox when the chain derailed.Total cost about $760
snapped teeth off 3/4 gear and the primary driven gear

The engine was put together by Macleay motorcycles http://macleaymotorcycles.com.au/index.html only problem That I can fault was the mechanic didnt trim the case gasket as seen below. Labor for pulling engine apart putting new gears seals and gaskets in putting it back together $180

got the spares from Scotties dirtbike supplies in Melbourne http://www.scottysdirtbikespares.com/
great service great guy,knows Gassers inside out.
Should be a goer midweekB)
while the bike was out of action I also completley striped the linkages and swing arm and steering head bearing and anything else that moves and cleaned and regreased it. 
ollie