nice work
now whats caused this is a lack of luberication, as the heat around the exhaust port burns up the oil causing a dry spot on the exhaust bridge . the barrel appears to be ok but a closer inspection will be needed make sure there is no bits of old piston stuck to the side of the barrel
with all my blata engines i always drill a very small hole in the scirt of the piston in 3 spots right behind the exhaust bridge which is where the damage to your piston
these holes will have to be around .5mm and no larger than 1mm and must not purtrude into the port they must be behind the bridge in the exhaust
now when drilling be carful not to hit the drill arbor on the piston then u will need to clean the burs created by the drill up with some sand paper ( around 1000 grit) untill there is no burs
then when reassembling ensure u use plenty of oil on the piston and ring
and when mixing your 1st tank of fuel even running the oil fuel mix at 25:1 any questions just ask