By explicitly making a column an IDENTITY column it is going to make a sequence behind the scenes even if one with a similar name exists. I tried and it created part_tab_part_id_seq1. Is there no way to have it use the original part_tab_part_id_seq?
No, the fact that there is even a sequence is mostly an implementation detail you shouldn't be concerned with.
How do I get the data and the sequence in sync?
That would be why the alter table command has a bunch of keywords and values as part of it. So you can modify the values to be what you need.