That small-sh spring in the first photo, do you know what it is from? We had a problem with one of our engines (before anything thinks that I am crazy for using the Royal "We", I mean several of us in our club that do alot of conversions, always great to have a community of nut cases ) when we converted the carb for use, we neglected to remove the ball and spring when we removed the choke butterfly. THis is inset along the choke axle, both the ball and the spring ended up being sucked into the piston and causing much the same damage that you have. This was on a 33cc homelite chainsaw motor. Just a thought.......you should feel priviliged, I only get one thought a day and I am sharing with you