The Q&A below is taken from EAA's website. It would seem to say that whether it was 90% (or 100%) built by someone else, you'd need evidence that the builder was an amateur and that they did the work.

The 51% Rule: Frequently Asked Questions
Q: If the kit is 90 percent finished by a previous owner, so I don’t do most of the work, can I still certificate it?
A: Yes, again the requirement is that the major portion is constructed by amateur(s) for their own education or recreation. However the burden to prove that the previous 90 percent of the work was done by an amateur solely for his or her own education or recreation is on the applicant. So one needs to be ready to show evidence in the forms of logs, pictures, videos, and affidavits that the previous builder was an amateur.

Here is the page this section was taken from:
http://www.eaa.org/en/eaa/aviation-c...uilt-airplanes