The gear should just slot in the way it came out. The chain should be visable through the hole. I think it worked best with the bike on the centre stand for this one. You will need a long screw driver to lift up the chain onto the top of the cog once its in. When in the slot it rides in just rotate the cog onto the chain. The chain won't move it just rolls on again.The starter chain is held in place further up in the engine by guides and these would not have come off. It's a lillte fiddly but I did it following a bit of swearing never to do it again. The re assembly is with the bike on the side stand. This allows the cog to fall all the way to the left. Slot in the rebuilt starter and wiggle in the spline the motor just turns to allow the spline to line up. Don't hit it to drive it i just leaver with a screw driver or something. It itsn't that tight. If it is it is because the spline is not in the gear wheel take it out and try again. make sure the o ring is good.
mark3, the hole is just large enough for the gear to slip back into the chain by starting one side and then the other. Never heard of one just popping out when removing the starter. If it came out and was still on the starter shaft when you pulled the starter you have another problem. No biggy toremove or replace. It just sits inside the chain when the starter is removed. (normally) Not sure if there is a front or back to the gear. Just don't push the chain completely to one side and drop the gear into the case.:waving:Keno