If you cut it down to that size.. your engine will probably stall.

Thepurpose of a flywheel is to smooth out the power coming from a reciprocating engine and to help it keep going.

What makes the piston compress the gases when it does??
The FLYWHEEL!
If you cut it down, your engine will stall easier and the power will be more "jerky" and raw.

Also your flywheel has to be perfecty true. If its not, the vibrations will probably kill your engine prematurely.

Getting a lighter flywheel will, however increase your throttle response. Not much but its something.

ram