How do I replace the timing gear & shear pin on a B&S 125K02-0658-E1 engine.

I have a Craftsman front propelled mower 22" that has the 6.75 B&S engine. I recently hit a tree root, and bent the blade. I took it to a local small engine repair shop recommended by your web site (Sears was too long a wait). They replaced the blade and checked out the whole system. After I got it back it started just fine, but ran a little rough. When I ran up on thick grass it seemed to knock alittle more than before I hit the tree root. After shutting down to empty the collection bag, It would not start. acted like a vapor lock. after letting it sit for afew hours it started just fine 4 Full Bags later It wil not start at all. The recommended shop feels it is the timing gear & or maybe just the shear pin. it rotates freely when you pull the starter cord (blade turns & Flywheel turns. Is there instructions online I can down load on how to do this? The mower shop (&Sear service ) can not get to it for over a month. They have that long a backload. Can you help?