I'd be cautious, but not regard it as a show stopper.
Having an aeroplane that old which is, in effect, a "new" aircraft built from bits from numerous sources is really not that unusual - the frame has been re-used to carry the serial number and give it an original identity.
So, the issue is the condition of that frame, which may in reality have been a new-build item by a competent fabricator. So, there's a a possibility that it's good as new.
And there's a possibility that it's extremely dangerous and fire damaged.
If the second, the rest is fine, it needed re-covering anyhow, and you can get hold of a new frame to transfer the data plate onto, then this might yet be a bargain.
If you can find a good licenced engineer, who knows welded steel spaceframe structures (and in an ideal world who is also a qualified welder) they should be able to look over it and give you a useful opinion - they can probably also tell you how much work the rebuild will take you!
G