Geoff, 28mm diana seals with a suitable piston nose adapter threaded onto the piston spigot make for a nice, refined firing cycle, and give you the option to increase stroke - well they do on an airsporter, as there is about 7mm of clearance from the cocking slot in the piston to the cocking shoe, but you'd need to measure the merc's clearance first. But even if you keep the stroke std, it'll still be a nicer firing cycle and less hassle than the overly complex BSA arrangement.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.