Those would be federally illegal: once a machinegun, always a machinegun.
Unless you are talking about people who buy parts kits and build them up. In that case, it's a grey area.
There are some parts kits that are so rare, they're so expensive that only a die-hard collector will buy them to begin with.
Then there are ones where everyone has them: amateur builders and collectors alike (think Romanian "G" kits). This is why I call it a grey area: some guys will build them into a 1:1 clone of the Pm. Md. 65 as used by the Romanian military right down to the receiver markings, finish type, rivet type, internals, etc., etc. Basically, only differing in the fact they are semi-automatic v. select-fire.
Then you get guys who bought a $200.00 parts kit to build a cheap AK that works. No one generally has a problem with someone taking an AK like that and modifying it, because it was just made to work and not to actually represent its previous, military service life.
Meanwhile, if someone did anything to modify this, they are a heathen.
I have no issues with people modifying their SGLs (AK-100 series "clones") because they are simply professional Saiga conversions, and aren't 1:1 clones of their respective service rifles, anyway. However, this guy took a Saiga and went way further than Arsenal/FIME by getting each detail down correctly to the specific rivets used.
It's kind of like people having no issues with people who modify, say, a modern 1911. However, if the 1911 happened to be a Singer from WWII or perhaps a new, limited production one that was built identically to the originals for collective purposes (like how that Saiga was built into a 1:1 AK-103 clone), then you are really doing it a disservice by modifying it.
You're ruining the one because it is a military surplus item: a literal piece of history, just like Mosins, SKSs, M1 Garands, and stuff like that. A soldier actually touched and fired this gun. Likewise, you're "ruining" the other one because it is a limited edition model that is an exact clone of the originals made as a tribute to the originals.
Basically, there are two categories of "don' touch," as far as I'm concerned: legit milsurps and commemoratives.
A run of the mill AR-15? GP WASR-10/63? Saiga sporter? Universal M1 Carbine? Rock Islan Armory 1911? A made-for-commercial-sales Type-56 SKS? Etc.? Doesn't really bother me if someone modifies them, to an extent.
That extent is basically: adding superfluous crap and then pretending it actually improves the firearm. If someone adds a dozen foregrips and six red dots "for tha lolz" and it's obvious that they did it because why not, then sure, whatever. At least they know it's stupid.
When someone adds Tapco everywhere, a $25.00 red dot on a mount that won't hold zero, etc., and then fights tooth and nail to tell you their SKS is now more effective than an RPD (I've seen people say this), they are a flat-out retard.
Then, of course, anything modified with purpose behind the modifications (I believe /u/Ruhlmdc used my AK as an example) is good-to-go so long as the items chosen are of quality. After all, as soon as you slap some accessory onto your firearm, your firearm is only as good as that accessory.