Captain,
As you can see from the pictures, there is a lot of space between the crankpin and the backcover. You can grind down the crankpin, about 1/16" and still have 0.020" exposed. You will need to tape the portion that you want to remain untouched. Dip in water often to retain hardness. You want to keep it very cool. Remove rough edges. You can fill in the space between the crankpin and rear cover by adding an aluminum disk or cutting down the rear of the case, or both. The disk is held in place with two screws through the rear cover tapped into the disk. Allow about 0.20" crankpin clearance. Remember that this must be vacuum tight. I thought that the crankpin mounting boss was square, but I was wrong. The piston has two webs below the wrist pin. These can be removed, since they contact very little cylinder wall and aren't needed to seal anything. This will lighten the piston and increase the cross section area of the transfer passages. The timing of a stock 25cc is: exhust=150 degrees duration, transfer=116 degrees duration, intake 120 degrees duration. These are actually typical of the more powerful chain saws and most glow engines. Of course the glow engines have rotary valves which open earlier providing greater duration. The last photo os of a Husqavarna saw piston with the webs of the Homey.

Dave