Xtreme Shooting Center sells WX-MCR receiver sets and propietary parts kits. The receiver set is showing as out of stock, but I'm sure you could talk to them further.
True North Arms has the parts to make a WK from scratch, including the steel upper with an alternate, non screw in charging handle.
SBI has indicated they are working on 180 uppers to compliment their lowers, with both piston and direct impingement versions.
Assuming they don't get delayed again, the Jard J180 build kits, using a 180 recoil recovery system directly gas impinged, should be imported by Sylvestre Sporting Good later this month.
