For AR series rifles, a registered auto sear is a DIAS. It is the machinegun.
By a complete sear gun, I assume that you mean a registered receiver gun. It is not possible to "cannibalize" one of these and install the parts in a different gun. In these, the receiver (complete with the hole for the auto sear) is the machinegun.
The '86 ban does completely ban the manufacture of full auto weapons for civillian use (the ATF will not approve the manufacture, and they can not be added to the registry). You got it right when you said that the sear is the registered item. It is "the gun." No new ones can be made for civillian use.
It is not legal to take an auto sear from a registered receiver AR rifle and install it in another rifle. The receiver is registered, not the sear.
They only way to legally do it is with a registered DIAS or Lightning Link (or purchase an origianl Armalite AR-10 (not one of the new manfactured guns).
"Those who live by the sword are probably pretty f***ing good at it."
"Instructions for a successful gunfight: Front Sight, Press Trigger, repeat
as necessary." - B. Braxton