Scrape the coverlay off the original flex pcb so that it can be soldered to. Be careful not to damage the traces as you don't have much to work with.

Join the new flex pcb to the stubs. This was more difficult than I hoped, as the solder would not bridge across the 0.07mm height difference reliably. The technique I used was

Use the double sided tape to attach the stub to the long tail of the splice PCB.

Use a much cooler soldering iron than usual, so that the solder can be 'moulded' as you push it around with the iron tip. If you get the right temperature it will be much easier.

Bridge the outer traces and the guard sections of PCB so you can use a large blob of solder here, which will bridge the gap between the two PCBs. Note how I've cut a piece out of the guard section so it doesn't join the motor and sensor.

On the inner traces use a fine strand of wire to join the sections.

Insulate the soldering with polyamide tape and reattach it to the body using double sided tape.

Check with a multimeter that the traces are connected and not shorting. Note that Pins 5 and 7 are one motor winding, and pins 6 and 8 the other, so they will measure as connected, but as they have a trace between them you can tell it's intentional and not a soldering short.