These engines have a 'synchro-balance' system fitted to most engines (some manufacturers for some strange reason option their engines without it to save money) which you should have seen when you had it apart.
Synchro-Balance consists of two big cast iron weights which are eccentric on the crankshaft held together by a long 1/4 unf bolt and small conrod to mounting on the inside of the crankcase. Was it all there and in ok condition?
Also check for fatigue cracks on the chassis itself where the engine bolts on
If you decide to change the flywheel you should be able to pick up a good second hand one fairly easily but make sure the ring gear is ok and of the same material you already have (they are either plastic or metal ring gears)
