While I have never seen a parts set, it is my understanding that the IO system is substantially superior to either of the SAS designs. The SAS tends to squash springs, and requires tweeking and balancing spring tensions.
A comment was made about ammunition for Stens.
Original carbines require good ammunition with strong cartridge cases. A lot of the casehead is hanging out in the open air, because of the coned breech. Thinner cases can rupture. Speaking from experience, this will get your attention! I suspect that a lot of problems with early Stens was related to the Winchester ammunition that was initially available. Blown caseheads can result in a bullet stopped in the barrel; clear the stoppage and fire, and a bulged or cracked barrel results.
The reproductions currently being made in the US are closed bolt, most firing with linear hammers. This is a very different critter than the original open bolt API guns.