Locomotive Flywheels

Here are a few questions I have about flywheels in locomotives, N scale of course.
I do not have any specific locomotives in mind, therefore I do not need specific information.
I am looking for a general knowledge of the subject.
I understand the physics of the flywheels and how/why they add momentum and inertia to the operation of the locomotives.

Can flywheels be installed in locomotives that were built without them?

Can additional flywheels be added to locomotives that do have them?

Can the flywheels be replaced with denser materials such as lead or tungstan to make them heavier?

Can the flywheels be carefully milled or drilled to remove some weight and still be kept in balance?

Is there a practical method of adding weight to an existing flywheel and keep it in balance?

Is there a way to add flywheels to other rolling stock to add momentum and inertia to a train?

Have fun.

Moose