The NFA defines a machine gun as "any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger". What level of work is required to keep a gun from falling afoul of the "can be readily restored" clause?
Gun University has a lengthy article on the topic. They point out that there is no precise definition for "readily restored," but there is case law. They cite United States v. one TRW Model M14, 7.62 Caliber Rifle from William K. Alverson:
Just as the GCA does not define “readily converted,” the NFA does not define “readily restored.” However, the term “readily,” with respect to the NFA, has been read by courts to “encompass several elements of restoration:
(1) time, i.e., how long it takes to restore the weapon;
(2) ease, i.e., how difficult it is to restore the weapon;
(3) expertise, i.e., what knowledge and skills are required to restore the weapon;
(4) necessary equipment, i.e., what tools are required to restore the weapon;
(5) availability, i.e., where additional parts are required, how easily they can be obtained;
(6) expense, i.e., how much it costs to restore the weapon;
(7) scope, i.e., the extent to which the weapon has been changed . . . ;
(8) feasibility, i.e., whether the restoration would damage or destroy the weapon or cause it to malfunction.”
They then point out how the line varies from court to court. The courts do not always even focus on the same elements of the list above.
They do mention that, if you need to have a reliable determination, the Firearms Ammunition and Technology Division (FATD) is available to assist you on a case by case basis. The article suggests they don't offer sweeping guidance, but will look at a sample item and make a judgement on it.