As Andrew said except it may be possible to restore the energy by loading the outer rim with Lead or Tungsten Putty.
Could be more trouble than it's worth. The centripetal forces on the rim of a flywheel are substantial and it's not unknown for them to explode, cast iron being notorious. The problem is making sure the added weight won't fly off, which probably requires an external rim, or drilling a large number of holes to contain the weights. Making a flywheel from full sized stock avoids such complications.
The extra width needed to compensate for a smaller diameter as suggested by Hopper could be considerable. As the energy in a flywheel is proportional to the square of the wheel's diameter, so the width has to be increased by a distinctly greater weight of metal than is lost due to reducing the diameter. Assuming that is the reduced flywheel must contain the same amount of energy.
Always good to ask 'does it matter'? Building a hot and miss engine is on my to do list but I've no practical experience yet. Flywheel size wasn't critical on the small stationary engines I've built – the engines run OK with flywheels smaller than specified in the plan. No load, so their flywheels only needed to store enough energy to carry the valves past top dead centre. Jason is far more experienced: are hit and miss engines sensitive to flywheel size, or does the builder have plenty of leeway before they refuse to run?
Dave
Edited By SillyOldDuffer on 08/10/2019 14:53:48