My first Norinco SKS got dressed in a Combat Exchange (?) Folding stock, detachable 30 rd mag, 4X scope etc.
Sold it to a deputy who wanted a carbine for his patrol car as a supplement to his issued shotgun.
Found out much later that my "improvements" made it illegal!
Look up the 13 (or 14 for European SKS) parts that have to be changed to US made parts in order to change it from the imported condition.
you do not have to change out 14 parts on an SKS to make it legal 922R says you can't have more than 10 foreign made parts and lists specific parts.
if it's original configuration then it gets around 922R by being considered C&R but as soon as you replace 1 part you have to change out enough with US made parts that it becomes compliant.
since the SKSM(SKS that take AK magazines) were made by century(or whatever company imported them) they had to make the SKS M fully 922R compliant before selling so adding the stock change OP is well within 922R compliance parameters.
normally with Chinese SKS all you need to do is replace the magazine and stock to make it 922R compliant as this removes the floor plate, magazine body, stock and handguard and drops the number of foreign listed parts down to 10.
as to the OPs dilema, I personally would not sell, if you still like it, shoot it and have a use for it then there is no reason you should feel obliged to sell.
SKSM are hard to find anyway and would a pain to replace.
is a page that lists the specific parts that can be changed to make it compliant.