Depends on what the DAR is willing to accept. I could paint out the N-number on my Fly Baby and claim it's a new homebuilt that I assembled from portions I purchased (which I did, in a sense). A DAR is likely to balk, though.
They're probably not likely to balk at a set of tail surfaces and some other minor bits, but each DAR will draw the line at what he or she is comfortable with. If major portions of the aircraft were purchased, and the aircraft type is the type that tends to have "hired guns" involved, one may be queried for more details.
Remember, much of this is up to the judgment of the DAR. It's no fun to get to the inspection stage and have to "shop around" for a new inspector because the first one refused to sign you off.
I'm potentially facing this issue. I'm looking at building a new Fly Baby to both avoid the ADS-B mandate (e.g., no electrical system) and enlarge the fuselage for more comfort (a concept I call the "Fly Baby Grande"). Ideally, I'd build a new fuselage (2" wider, aft cockpit bulkhead modified for more legroom) and graft just about everything over from my current Fly Baby (FWF, wings, tail, gear, etc.). I'm contemplating what I'll have to do to lay the groundwork for this....
Maybe I'll throw the question up to the FAA staffers...they'd probably LOVE a question that didn't deal with Young Eagles certifications.... :-)
Ron Wanttaja