Amin - Are you looking for the Canon 50/1.2 in LTM because you want the
particular lens signature (low contrast, very soft at full aperture, "interesting" bokeh), or because it is one of the least expensive fast 50's out there for your Bessa?
If you are looking for something a little more modern, you might consider this
Canon 55mm/f1.2 FL conversion. The last one this guy sold went for something like $320 (though I bought one from him for substantially more). Mine is an FD series and I like it more than the LTM Canon 50/1.2 I sold.
You might also consider the CV 50/1.5 Nokton - sharp, contrasty, great color rendition and flare resistance - and also relatively easy on the wallet when purchased used. Or maybe the 40/1.4 Nokton - you could make use of your R3As 40mm framelines. I hardly notice the half-stop difference from the f1.2 lenses - and anyway it is easy enough to push HP5+ to 800.
BTW - I'm expecting a Beattie screen for my OM-1 any day in the mail - looking forward to having a little more focusing light when using the 50/1.2 wide open.