Yikes, I guess the plugged muffler might be what if from the engines dismanteling itself.
I had taken old Briggs lawn mower engines and they needed carbon removed and the valve and seat angles recut, but no missing metal other than cylinder wear. That wear didn't even need new rings.
The scrape on the side of the piston I'd say it would be lossing a lot of pressure.
Having fun riding though is what it is all about, get it running again soon