Simply drilling holes in the right place and installing the flapper valve and shaft would probably convert the Fenton to have the heat riser function. Press fit a couple of steel bushings into the hole to support the shaft and add the snail shell bimetal spring on the outside. I've fabricated heat riser flappers and shafts from stainless steel- - - -they last about 3 days past forever.
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!