If your dummy pump can be dismantled with screws between the two housings, just replace the diaphragm with a piece of soft aluminum sheet metal and remove the lever arm. Install a blind gasket at the crankcase hole. No flexing to worry about and the engine will look original from the outside.
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!