Try fabric paint first, the blinds do not have to look nice, the photos will be ok. If a ribbon has snapped then you need new blinds.

If you must fix it yourself, then a beater Fed I or Zorki I is a good training exercise. Then you can take take good photos with them while you have your IIIf in pieces, sell the little camera as a just serviced.

You will need to build a shutter tester to confirm that the gap in the blinds, is sufficiently uniform, this saves a lotta time, even if you can accept a lot of acceleration across the frame.

You will need to adjust the register to recollimate, and then the rangefinder, unless you are real lucky.

Google will get you all the detailed things you need to know.

I normally wheatstone a/each screw driver so it just it just fits each screw type, as damaged screw heads will put off customers. Take notes and photos as you strip. Dyson the work area before you start.