Use masking tape to hold things in place while you work on them. I also cut some pieces of rigid foam to hold things in position. I recommend mounting this on a board that you can rotate for soldering at convenient angles. Cover the lenses (I used pieces of ziplock bag taped across) to avoid flux splash or dust while working.

Use a scalpel to scrape away some of the epoxy coating, being careful not to cut the copper traces.

Use some 34AWG stranded wire to replace the damaged section to flex PCB.

Use a multimeter to check the best you can, as you don't want to have to disassemble it again after testing.

Clean with flux remover on cotton swabs. Followed by IPA. Insulate with kapton/polyamide tape. Use double sided tape to reattach cables where they need to go.

Soldering the ribbon was very difficult. The traces were ~0.1 - 0.2mm wide. Stagger the soldering positions to help avoid shorts. At this stage I'm giving this repair job a 20% chance of success. If this doesn't work I will manufacturing a new flex ribbon - although I fitting to the motor will be challenging without removing the calibration screws.