Hmmmm! As I continue to think about this, I'm guessing that you could simply remove the spark plug and see if that caused the engine to crank normally. If it does, it would lend support to the suggestion that you have an engine problem. If the engine continues to labor with the spark plug removed, probably NOT that suggested engine problem. Wouldn't that be a simple, useful test to try before tearing apart the engine?
What other tests can you devise to help identify the actual problem rather than guessing and investing in making a repair which may not help?