Since the receiver is the only registered part, you can legally add a parts kit to a registered receiver to make a fully functional machine gun. No special permission from ATF is necessary. If it's a C&R receiver, the result will be a C&R gun; that's true also for a rewatted C&R receiver. Otherwise, it will be a non-C&R "tube gun"; that's also true if an otherwise C&R receiver was cut and rewelded.
All transferable machine guns (receiver or sear) must have been registered with ATF by May 19, 1986.
BTW, parts kits for Sten MKII's are not as common as they once were.