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You don't know the steel, so it is a guess work quench/ heat treat. Best hope is for simple high carbon tool steel

Really you are not re-tempering. the temper is gone when overheated. Which is...

probably not explicitly possible. edge will burn up, then there's scale. Assuming your piece survives heat treat it will need regrinding so it will be a fair amount smaller .

just so you know; you...