On a chrome bore block, a lot of the time what you are seeing as scoring is actually aluminum transfer from the piston. That is why you take it apart and clean it with muriatic acid. Muriatic acid will eat the aluminum away and could leave you with a perfectly fine bore. Muriatic acid does not...
Look for carbon plugging in the exhaust. The earlier model blocks may not have the Bung for staking the stator in place you will need to look at that. Stay away from the non thermostat models. I personally would start with tearing yours down first before anything.
Forget about trying to get the compression up the motor needs to come apart. After it is apart take Muriatic acid and clean the bore to see if the chrome is flaking off.
Because of Emissions the carbed motors are jetted extremely lean at an idle, that can cause the problem you are having. They are extremely easy to plug up, might be time for a carb cleaning. Or your idle is to low, Or something you have to live with
Here is the correct # for a chrome bore piston 39-17286A12 which supersedes to 39-827491A12. So I am going to buy a used Mercosil piston and compare them. If this is true I am thinking you should be able to use the WSM kit for the Mercosil motors to rebuild the older chrome bore motors.
