The only LEGAL way forward if you insist on the "New" frame is to get a proof of year of build letter from HONDA (about £30 and 8 weeks) then rebuild into new frame, mot and insure on frame number then pay the registration fee and tax cost to get a new "Age Related" registration.
Personally I would cut out the rotten tube and replace with either a donor one or a fabricated one as this requires no notification whatsoever to anybody