Are the drive pin and collar available as separate parts? Can you tell me if the pin and collar are available as separate parts or will the whole driveshaft require replacement?

These parts are not sold as a separate item only as part of the assembly. Here is the part: https://www.lsengineers.co.uk/drive-shaft-for-stihl-km56c-km56rc-part-no-4180-710-3200.html