Are parts or kits which would convert a firearm into a machinegun subject to registration?

Yes.

[26 U.S.C. 5845(b); 27 CFR 479.11]

Source ATF: https://www.atf.gov/firearms/qa/are-parts-or-kits-which-would-convert-firearm-machinegun-subject-registration

Other NFA Gun Trust and ATF Related Blog Posts