It's hard to imagine that undelivered aircraft, for which Boeing still has responsibility, are being neglected to the point where they would need remedial work (over and above the post-MCAS mods) in order to return them to an airworthy state.
Periodic maintenance of an aircraft that's long-term grounded isn't complicated - the Maintenance Manual details the tasks that have to be done, and how often. I'm confident that those are being carried out, either by Boeing or on its behalf.