I am sure that someone smarter than me will be along to answer this with more certainty. But, I believe if you start making changes, you will probably want to make sure you have the right amount of US made parts. I believe you will need (5). The fixed stock, if you get a US made one would count as one part. You could then change the pistol grip to get two. From there the gas piston, trigger group (each part counts) and the upper and lower hand guards are candidates. I think you can even use US made parts in your mags (springs, followers, floor plates etc.) to reach the count. But if you use mag parts you would have to make sure each mag you use has been changed.
Now with all of that being said, I will confess I don't know all of the ins and outs. The above is just my limited understanding so far.
I am also curious, because I have been thinking about one of those same rifles. Like you, I would want the fixed stock.