Can a pilot or owner repair a seatbelt? I have a Cessna 210 with inertia reel seatbelts.  The spring coil on the retention wheel broke.


FAR 43, Appendix A (C).14 allows for owner replacement of seat belts 
FAR 43.13.(b) requires that it be restored at least to its original
or properly altered condition.     
and FAR 21.9.(a).5  allows a part produced by an owner / operator to be installed


so... can I as owner / operator:


repair the spring and re-install the seatbelt
or
purchase a used seatbelt with a working coil spring (but no good webbing) and replace the coil spring?
You cannot repair a seat belt. You can only replace it.

An owner-operator can perform preventative maintenance on their aircraft. That includes replacement of the seats and/or seat belts.