There will either be a tag riveted to the front edge of the blower housing, near the bottom. Or a spot where one used to be. During the time period of your bike they had an aluminum tag that either corroded off or simply fell off. They used a very poor "push" rivet to attach it. Hopefully yours is still on it. If it's a Tecumseh plate we can date the engine to the day. If it's a sears you can't date them as far as I know of. If it's missing you will never know.