You might check with a machine shop but I know they turn and use flywheels that have heat cracks in them all the time.
Yours doesn't look to bad to me but then again I have never owned a 4 spd car before. I have seen pulling tractors with turbo motors run ones that have a LOT more cracks in them then yours.
The flywheels are pretty thick.
Just my 2 cents. David