The 20 rounders (I am assuming they are the ones that look like an oversize stock 10 rounder?) are hit and miss. Sometimes they do work, and sometimes they don't. Usually the sporadic ones have a star on the side. "Back in the day" there were models available that were not the stamped star variety that worked really well. These are hard to find now.
I don't think any one is actually performing a conversion for the SKS to accept AK mags per se. The ones you may have seen are Chinese SKS "M" and "D" models that were actually produced this way from the factory. They work really well, but since there aren't as many in existence, they bring a premium (usually $300.00 and up). At that price, you may as well get an AK, unless you want one just because you can.
There are plans floating around the net to do the conversion yourself on a standard AK, but unless you have access to a mill, the results may not be satisfactory.