It appears to hinge on the definition of 'modified'.
You cannot modify a gun such that it is below certain parameters. But.. does replacing a factory stock with another factory stock constitute 'modification'?
Apperently not. And that makes some sense, considering that this would mean everytime someone replaced a stock the gun would become prohib'd if it were below the minimums, even if it started out that way.
When you look at the law that way - there is clearly not a violation. Sawing down an existing stock might be a modification, but replacing a stock with another factory made stock is not. It obviously did not occur to them to consider pistol grips when they wrote the law. They DID consider folders, which are definitely out.
So with that in mind - as long as the barrel is factory made and the stock is as well, the law doesn't forbid it. It's relatively simple when you see it from that angle. In the past we always assumed changing a stock would be 'modifying the gun' in the eyes of the law, but obviously they didn't see it that way and it does make sense. It's not a 'modification' to install factory parts. Only changing the barrel length is expressly forbidden.