How difficult is it to replace a flywheel on a Tecumseh vertical shaft
engine and are any special tools needed? We have one which has a couple of
bad sections of gear teeth (used by an electric starter). I have no idea
why as the starter gear is plastic.

I'm presuming the gear teeth are integral to the flywheel and therefore
metal, unless they somehow add a ring around the outside with the teeth on
it. (That would help explain how it could be damaged.)

I can go fetch the engine model number if need be though I think most of
these engines are very similar. It's on a Lawn-Boy walk-behind mower.