I've adapted a Kiev TTL Spot prism to the S2a, so I suspect that a Kiev focusing screen might be about the right size, but you might have to cut it down a bit. One question is where the focus plane is in the screen. In the S2a, there is a plastic fresnel lens facing upward that rests in a well below the focus plane, and then the groundglass sits on rails, matte side down, and this is where the focus plane is. There are springs pressing up on the groundglass, and there is a frame with polyurethane foam pressing down on the groundglass. When this foam deteriorates, you have a focus problem, but that's another story (an easy fix, but it seems to happen to all old Bronicas).
So, if the Kiev screen has the groundglass surface on the bottom and the fresnel lens on top and if it is large enough to sit on the rails that support the groundglass (perhaps after cutting it down slightly), then it could work. If the fresnel is on the bottom, though, then you would have to do something like cutting a rabbet all the way around the groundglass, so that the matte surface inside this one-piece groundglass/fresnel screen would be in the right plane.