Quick lesson :
The small metal piece that sheared is called a key .
The groove the it fits into is called a key-way .
Your cam wick looks dry . It should be oiled .
As for replacing the crank seal and anything else on / about the bike , here's a link to a service manual . You should read and understand it well .
Lapping flywheel to the crankshaft :
Apply lapping compound to the taper . Then the flywheel goes on without a key and gets hand turned a few in one direction , then the opposite direction as many times . Repeat until surfaces involved are showing smooth and matching . The idea is to create as much friction as possible between the crankshaft taper and the flywheel taper . (edited)