For your flywheel system to work, it would have to be heavy and probably more mechanically complex than the rest of the bike put together. And for it to be worthwhile the power the flywheel provided would have to be more than the power used up in simply bringing that amount of weight up a given hill. Personally I think batteries are a better way to store power (despite all their inefficiencies) than flywheels, and flywheels are never gonna become a good power storage mechanism for small vehicles. But I might be wrong.