I had a x39 and 'unconverted' it.
It can be tricky, especially if you don't have the proper tools. The trigger guard is made of something akin to titanium, as I had a hell of a time reshaping it so it would fit under the mag release. Required some time on a sturdy vise and 5# hammer.
Drilling/tapping the bullet guide takes patience, but not particularly difficult. Drill the hole, lube the tap constantly and only turn 1/8 at a time! Back off, then another 1/8 turn. The thing will snap off if you get too excited.
I had the most problem reinstalling the BHO. Later on I discovered a trick to lining everything up, but still took me about two hours getting that sucker to go back in. Some people just leave it out though as it's not a standard AK piece.
Tools needed:
Dremel with cutoff wheels
vise
tap handle
drill
screwdrivers/pry instruments
Check out the Saiga-12 board for everything needed. I went with Dinzag for my trigger and bullet guide kit.