Check RTG for the missing parts.
http://www.robertrtg.com/ He may have what you need in the small stuff. Good people to deal with. Their price on the MG42 tripod (with optics!)is also very good right now (never likely to be cheaper).
Barrels are scarce but not impossible to obtain. I would pick up what ever you find, as you find it. They are not cheap, but buy it when you see it. The lesson comes from .50 cal ammo - I thought it was expensive, but in hindsight it wasn't and now NONE can be found.
Also your current barrel has value to those building dummy guns. Selling that (currently I wouldn't) would offset the cost of a better one.
In your position I would get on a waiting list for one of those new receivers. When they come out you'll get one OR, you'll be waiting for everybody else to get theirs, the mfr will catch up and FINALLY you'll get one (at the new, higher price).