I was not sure what you were referring to. The foreign parts thing is usually a reference to complete guns. Who cares what parts you put into a gun? No one.Who is checking foreign parts compliance?
As for the legality of SBRing the gun in question, it will still need to be over 29" OAL, even if the gun was owned prior to 10/2013. When you SBR a gun, it becomes a new gun, subject to current regs, as long as it remains in the SBR configuration.