You would need to remove the manifold and either install machine screws to hold the flap valve to the shaft and peen the ends of the screws to keep them from backing out, or tack-weld the flap to the shaft. Be sure to position the flap properly before securing it to the shaft- - - -it's going to be a one-time opportunity to get it right. I've fabricated flap valves and shafts from stainless steel for a long-term fix.
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!