About the repairman certificate: AC 65-23A ( change 1) para 4(a), applicant must attest to building more that 50% of the aircraft, which must be notarized.
Para 4 (e), Present satisfactory evidence such as construction log book, pictures, drawings. Para 7. certificate should be surrendered whenever A/C is destroyed or sold, however in the latter situation the repairman may elect to retain the certificate in order to perform condition inspections for the new owner.

AC 20-27G appendix 8 is used as a work sheet to calculate the 51% and should be presented to the DAR along with other evidence of being the builder.