Alternative solution- - - -unless you're getting a noisy exhaust leak, ignore the sloppy fit. Carbon buildup will cause even a moderately close fitting shaft to lock up pretty quickly. If you absolutely must replace the shaft and flap valve, drill and tap the manifold for brass bushings made from 1/4" NPT thread fittings, and drill and line ream the holes for a stainless steel shaft and flap valve. I've reworked several heat risers that way. That's the time consuming, E$$$$PENSIVE way to do it, but it will outlive your grandkids.
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!