You could always drill a small hole, screw a sheet metal screw for a slide hammer in, and weld up the hole and re-use the plug for reassembly.

Last resort, just turn one out on a lathe with a magnetic chuck. It would also be possible to set the case up in a Bridgeport mill and counterbore the hole a little to create a step for a flat dish-type expansion plug.
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!