12.5 is getting high for the gas you get today...especially in an air cooled motor. Milling the dome is an option, as well as opening the combustion chambers up...just be sure to measure and equilize all 4 chambers. You can also use thicker head and base gaskets. Timing can be backed down a bit to help with the pinging to but you will loose some performance. A lot of things contribute to how much compression you can run before you get into trouble. Take a look at modern liquid cooled street bike...my CBR954 runs 11.5:1 and runs all day long on 91 octane...but it is liquid cooled, fuel injected, etc. My CRF450 motocross bike runs 12:1 compression and I run 91 octane and never an issue. If you do carefull measurements and do accurate calculations with volumes of camber, piston dome height, valve relief volume, headgasket thickness and deck height you should be able to get it to a reasonable 10.5:1 or so compression ratio. May be worth the effort in finding someone that would buy the 12.5 pistons and buy the lower compression piston.
