Looking at the Countach manual, you'd need modified versions of the A8 & L1 parts. These can be made on a small hobby lathe / mill or the flats could potentially be hand filed if your hand work is good enough. Accurate dimensions would be needed though.
It's possible but not in any way guaranteed the L3 part is the correct size, it is certainly the right nominal shape. I would certainly disassemble the axle & see if it fits in the wheel.
Otherwise you would nee the entire diff from an RM plus the wheel hub & even then I don't know if the axle diameters are the same.
