As above, I have a 1951 MK1 crankcase for sale, RD531.
It was removed from a running bike, and appears to be totally solid and servicable. Only damage is on the blind bolt holes on the sump plate, where it appears someone has attempted to put longer than intended bolts in, and have pushed through. I'm sure these can be easily welded over again to avoid oil drips.
If you are interested please PM or call me and I will send you some pics, etc. I also have the 2 main bearing shell retainer clips.