Pete,
You could try Jotika. They sell Aeromarine radio boxes. I think they come in about three sizes, and the ones I've seen are uniform depth (rather than deeper at the back) - which may or may not suit your application better. For the Makara which I'm currently building, I bought one of the largeish Prestwich boxes, but ended up turning it sideways - to get the rudder servo further to the right of the boat, and also so that it takes up less length (ultimately so that the fuel tank and engine can go a little bit further back).
Many make their own radio boxes - so that the shape and size are best suited to the boat. Thin ply is good. You can tack it together with superglue - then coat it with epoxy inside and out. This makes a strong box, and the epoxy should make the joins watertight - as well as sealing the grain. If you do make your own, try and get polycarbonate sheet for the lid - instead of acrylic (perspex). Polycarbonate is not quite so brittle.
I spoke to Ian (of Ian's boats) a while ago, and he said he'd sorted you out with a CMB header for the Picco. If you're still struggling to identify the engine, I remembered one clue that may help, but only if it's a Picco .80. The .80 has a recess cut across the top of the backplate - so that the piston doesn't foul the backplate at the bottom of the stroke. The smaller engines don't have this, as they are shorter stroke. If you take the backplate off to check this though, make sure the crankpin goes back into the right slot in the rotor disk - when you reassemble.
I also asked Ian if the old purple head CMB 45 CAM crank (like your broken one), has the same dimensions as the current CMB 45HR crank. He will try to find out.
Ian