Welcome to the group!
If it was originally an underfolder, the "slant cut" is normal and it should have the underfolder cover over the rear of the receiver with a stock tang welded on. My 2 cents is to find a complete Norinco underfolder and install it. If you don't want an underfolder, I suggest selling it to someone who would be more than happy to restore it to it's full underfolding glory.
Seriously, if you must have a fixed stock you could cut the rivets, remove the rear cover and install the rear trunion for a fixed stock which will not fit correctly and give you a rifle with almost no resale value.
So as a collector myself, and although cutting the rear tang off the folder receiver cover undoes the original post-Bush1-ban rifle, it seems acceptable in only that one type of AK variant that doing something that can't easily be undone does not really hurt it's value.