Power (torque) come from engine. It is'nt altered by flywheel.
Flywheel is a rotating mass that can store momentum. It has two effects:
1) when engine is comming from a lower RPM to a higer RPM: Engine will increase rpm slower since it has an added mass to accelerate. It can increase traction due a less agressive behavior and it will decrease power burst you have using throttle.
2) When engine is comming from a higer RPM to lower RPM: Flywheel will give energy to system. Rotating momentum can be used to drive the motorcycle.
An side efferct is the engine runnig in a more steady way when travelling during a rough terrain like over a bunch of loose rocks or loose terrain. On malleability deppartment, it can increase the "heavy feeling" on motorcycle just like we feel on a big bore engine.
Had played with flywhhel a lot and my feeling is: It only good in specific conditions (mud, senior rider, MX bikes on enduro). https://youtu.be/16d-ewfc8lU For average Joe, most of time, better stick with what come from factory.