That mower needs a 25:1, as you already have soring, I would suggest not trying a 50:1 mix, however, the 50:1 mix will not stop it starting it will just damage it once it does.
I would suggest a full compression test, as you have already popped a spark plug in the decompression socket, now would be a good time.
Disconnect the spark plug lead, and flip the mower onto it's side, then turn the blade disk by hand, anti-clockwise.
You should feel 2 points of resistance (compression points) per revolution, one should be distinctly stronger than the other.
Now, flip the mower back up, and pull the cord, not to hard, just pull it out at bout 1/2 the speed you normally would, it should get very hard to pull at some point.
Let us know how hard it got to pull the cord, and how strong the 2 resistance points felt when it was on it's side, also, how great was the difference between them.
Also, is it a possibility you could take a clear photo of the scratches on the piston, when seen from the exhaust port.
