I've fabricated replacement parts on a few heat risers. The big problem is replacing the bi-metal spring, if it's weak or broken. Sometimes an automatic choke spring can be substituted. Don't make the shaft clearance too close, as it's an open invitation to rusting solid again if there's not at least a little slop in there.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!