there seems to be a gap in the market here... thinking a new pin, with a smallish (say m5) short threaded female hole in each end, with a bolt that clamps up for each side.
However I can't remember if the barrel grips the pin (above wouldn't work as it would unscrew), or the jaws grip the pin (would work fine)…
Perhaps easier is a nut and bolt that threads through a hollow pin - I think the remington express, or maybe it was a Diana (something cheap anyway) had this arrangement and it seemed to work fine..
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.