I don't think it's necessary to register an aircraft as experimental in
order to legally owner/manufacture parts for it. There's a section of the
FARs that allows for owner built parts. This section allows orphaned
classic and antique aircraft to keep flying.

The rule is interesting. Owners can make their own replacement parts. But
there are guidelines. My friend owns a Funk airplane that had rusty struts.
After researching the rules, he learned that in order to manufacture the
replacement part, he had to have the blueprints for that part. He could not
simply remove the old part and make a copy of the part. He had to locate a
place that had the blueprints, then purchase a copy of that part of the
blueprint that had that part on it. Once he had that, he could legally make
a replacement part.

Colin