Exactly - with the older bikes that have been ridden, the cog is usually very tight. I would make up a second driver with a spined cog of the size of your choice and use that for riding. The driver metal is hard and brittle - it's easy to damage (chip or crack usually) the tines of the driver, or gouge the clutch, if you're not careful trying to unthread the cog with the chainwhip. Kroil and warming may help with removing the old cog, but on some, they just won't come off. If you do choose to make up a second driver with a splined cog, you'll want to try to match the tine profile. Usually the 1950s-60s era drivers are close enough to match. There's a later (stepped) profile driver from the 1970s that may lead to shifting problems if you try to use it.