What certifications must one have to repair/alter propellers and instruments?
I believe you need to be an FAA Certified Repair Station approved to work on instrumentation, propellors etc. as noted here 


  The FAA lists and approves ratings on a form called operations
  specifications. The operating specifications will also be displayed in
  the maintenance facility, usually right next to the certificate. Like
  other businesses, most maintenance facilities advertise on the Web and
  in trade magazines, so you have many ways to research what rating a
  particular repair station may have. There are basically six ratings
  that pertain to a repair station:
  
  •  Airframe 
  
  •  Powerplant 
  
  •  Propeller 
  
  •  Radio 
  
  •  Instrument 
  
  • Accessory


To be an approved Repair Station the employees must conform to 


  § 145.157 Personnel authorized to approve an article for return to
  service.
  
  (a) A certificated repair station located inside the United States
  must ensure each person authorized to approve an article for return to
  service under the repair station certificate and operations
  specifications is certificated under part 65. (b) A certificated
  repair station located outside the United States must ensure each
  person authorized to approve an article for return to service under
  the repair station certificate and operations specifications is—
  
  (1) Trained in or has 18 months practical experience with the methods,
  techniques, practices, aids, equipment, and tools used to perform the
  maintenance, preventive maintenance, or alterations; and
  
  (2) Thoroughly familiar with the applicable regulations in this
  chapter and proficient in the use of the various inspection methods,
  techniques, practices, aids, equipment, and tools appropriate for the
  work being performed and approved for return to service. (c) A
  certificated repair station must ensure each person authorized to
  approve an article for return to service understands, reads, and
  writes English.


They also must meet the requirements set fourth in: 


  (b) Meets the eligibility requirements of §65.101.


which you can find here.

You can find the full regulations pertinent to becoming a repair station here