your problem lies there with the engine mount screws - either u forgot to threadlock the screws or they worked loose regardless. this caused the damage to the spur gear by the sounds of it. we have all done it im afraid.

i would replace the gear, and maybe get a couple of spares, as they dont last forever, and ensure the mesh is correct and the engine mount screws are tight and threadlocked once u have fitted the new spur gear.

all the info u need and tonnes more is under that search button - but give us all a shout if u get stuck.