If the engine has points, clean/replace them (and the condensor), or upgrade to electronic ignition (cheap and easier than you may think). If it's already electronic, then the coil will need to be replaced. At that age, it could be either.
What's the engine make and model, and is the coil on the outside of the flywheel, or under it?
phil_saunders(a)bigfoot.com
Phil Saunders, May 2012