Bob, I hope you replaced the key with a genuine Briggs one. They are made of zinc so they can shear when the blades hit something unyielding, like a tree stump or water pipe. You would probably have a broken crankshaft right now if the original key had not been made of zinc.
You need to put the flywheel on loosely without the key, then slide the key between the keyways in the flywheel and shaft with the flywheel in correct position. If you put the key in the crankshaft keyway first then lower the flywheel over it, you are likely to have the key move downward slightly and jam the flywheel slightly crooked, and with excessive stress on the hub. The flywheel is likely to come loose, run eccentric, etc. if you do that. You might even crack the flywheel eventually.
Because the key was sheared, something horrible has happened to the cutter disk, blades and drive. It would be a good idea to take a look at that system: a stitch in time saves nine.
