The finder on my IIIa is quite usable, not been CLA'd. I have had two Canon's and two Leica's CLA'd to clean up foggy finders, came back crystal clear.
The J-12 will go onto a Nikon RF, but it is tight.
I do not use the Contax much, prefer the Nikon S2 finder. But I would not let go of the IIIa, it just oozes precision. The F1.5 Sonnar is an all time favorite.