Removing sprocket
1. take off cover to opener (don't go further if you can't get past this)
2. Loosen tension on chain. Above door, loosen the 2 bolts that tension the pulley and inch or so.
3. take off 2 of the motor support nuts on just one side and let the motor hang on the bolts one the other side, you can now get wrench in to remove the nut above the motor inside the housing. Put the 2 motor mount nuts back on and move to the other side and do the same thing to remove the nut above the motor on the other side. Leave the motor hanging so the shaft is not engaged with the sprocket. Remove the bolts you took the nuts off of that secure the bracket on top of the rail.
4. Loosen the side nuts on the rail that secure the front of the bracket that hold the sprocket down.
5. I removed the wing nuts and spring off of the bolts on either side of the rail and slid the bolt heads down below the piece they stick up through.
6. Slide the bracket back that is on top of the sprocket. Once it is lifted off, with some twisting the top metal piece will come off and you can slide the chain off and remove the sprocket.
This info comes with no warranty and all that, I am nobody who knows nothing and not a professional by any means. I drink beer on the job. Good luck