I don't think any MF lens matches an 50mm f1.2 outside of maybe the contax 80mm f2 (645)or Hasselblad 110mm f2 (6x6), but both I think are incredibly expensive as well as the supporting cameras. For thinnest DOF for the money, then I think that you would want to go with the Pentax 67 105mm f2.4.

But personally, I don't think super thin DOF is necessary and MF bokeh is incredibly smooth. I have an 80mm f2.8 lens but I prefer a little more DOF so I shoot at f4-f5.6 if light permits.

Untitled by Michael_Sergio_Barnes, on Flickr

11 by Michael_Sergio_Barnes, on Flickr