Tecumseh Mini Bike Engine

Happy to be in an environment where I can learn how best to care for my minibike engine.  I recently found a "Needle in a Haystack". When I was a kid (11 years old), my brother purchased a minibike for me for Christmas.  Over the last 5 years I've been searching for one exactly like it.  I was at the point of giving up and low and behold, I was watching a random YouTube video someone posted of a Minibike Swap Meet and I saw a bike just like mine in the background.  I took pictures of the TV.  I joined the forum at an advertised website and noted my interest in finding the bike; I also added the pictures I took.  Someone responded with, "It's going to be really hard to find the exact bike.  You most likely will have to build it piece-by-piece."  And then someone responded by saying they'd only seen my type bike twice in the last 30 years.  He put me in touch with one of the sellers and we struck a deal.  I just received the bike last Saturday.  I'd like to restore it to its original condition, even though it's in original form. I can't believe it...it's been 50 years!  One of the issues with the engine is that at mid-speed, it vibrates like the devil.  Has anyone ever had a flywheel checked for balance?  It's the only thing I can think of that would cause the level of vibration I'm experiencing.  Thoughts?