I recall that the re-magezined SKS must follow the 922r rules on US parts count.
A mill is really needed to open up the SKS receiver (and stock) to accomodate the typical AK box mag. Some SKS receivers are soft some are rock hard, and some mixed. Go slow, use a 3/8 inch diameter carbide ball end mill for the most part.
Magwell corners will require a 1/4 inch end mill (or Dremel). The tricky part is the proper depth of ball mill cut into the SKS receiver. That cut places the magezine into the correct feed position. Go no deeper that the existing forward radius depth only wider. (make sense ???). The front of th AK mag feed lips register at that depth.
The bolt hold open catch must be removed (no spare room in there).
A horizontal slot must be milled at the flattened magwell front to retain the AK mag nose. The slot also partly cuts into the barrel face. A small woodruff keyslot cutter works well. Go slow. The slot is 1/16 inch above the bottom edge of the receiver. This feature is a good place to start you SKS surgery.
The new AK mag sits at a raked angle (vs the usual AK) but feeds that way in an SKS.
A mag catch must be fashioned to hold the AK at the rear perch. Some builders have brazed a small bit of steel to the top of the SKS catch to do this. Some have made an AK-like catch welded to the SKS trigger frame.
The SKS bolt must be narrowed on the bottom surface to pass between the AK mag lips. A grinder can do this using care.
The mill work could likely be done with a die grinder, carbide burrs, layout lines, and lots of patience.
Years ago gunsmiths would charge about $100.oo to do this conversion and that was cheap too !!! Today ????
There must be an old tutorial on the internet for this conversion.