Need help with a 1978 70hp Johnson. Turns over but no spark

That old system runs off a spark advance and point system. It'll be located under the flywheel. Clean the magneto where it comes up to the flywheel, the spacing can be set with an old cardboard loose leaf (school scribbler) cover . To remove the fly wheel you will need to take the pull cord off , unscrew the big but in the middle of the flywheel and get a flywheel puller. Under the flywheel you'll find a point system . Either try cleaning the contacts with sand paper or purchase a new point /condenser kit . ( usually the setting for spacing the points is 2 sheets of loose leaf paper between the point contact and condenser.