IMHO, mass is what makes the torque value of the motor. But the flywheell can be lightened at the outer edge only, and a heavier pulley put on the front of the crank, to make the total package weigh the same. It improved the acceleration rate. In all of the engines I've worked on, and I can't count how many that is, We always got the pistons to weigh the same, the rods to weigh the same and also end to end weigh the same. That weight was then totaled and the weights added to the crank journals. Then the crankshaft was spun up to 10,000 rpm, when balanced and checked again. Then the pulley, and the flywheel with the pressure plate on was checked to make sure that harmonics were directed to the main bearings. Take the whole thing to a professional balancer and explain what you plan to do with the engine, they will usually be only too glad to help. An old rule was that a gram of weight out of balance at 10,000 rpm was equal to a 187 lb side load on the bearings. Of course, all of this is just IMHO. I hope it helps, I really do.