Removing the fins- use a hammer- smack them quite hard and “snap” off they come. Trying to cut them was a pain, the hammer works well.
Sanding- put the thing on an engine and run- this does not work well- tried the engine running and ‘sanding’ with an angel grinder- no good. Maybe a more aggressive disk would have been better. Tried a sanding block- too slow. It may have worked but it was slow going. So what I ended up doing was pulling the flywheel back off, again, and using the angle grinder to get close. Don’t stay in one place long. When I was close to flat I switched to a sanding block to avoid dips. Now that it’s almost there I put it back on the motor to get a ‘final’ surfacing. Pull it back off and’polish’ on the bench.