I think that you should install this piston into one of your bikes and determine the following:
-Will you be able to get the head on and bolted down?
-With the spark plug out, will you be able to turn the engine over without bending a valve?
-With the spark plug in, will the starter motor crank the engine over?
-If the starter motor won't turn the engine over, can you kick it over without breaking your leg?
-If you are able to kick it over and start the engine, congratulations, and how does it run?
-If you made it to this point, I think your calculations must be off !