If you know what you're doing or can write them off you can open them and clean them, just take care not to damage any coatings.
Lens can trap moisture over time which can eventually lead to fungal blooms which eat away the coating on lenses, I've rescued some of my USSR Camera kit in the past but I'm not an epert by any means.
You'll Shoot your eye out Kid
Currently looking for an SMK/BAM B4-4 Rear sight or help modifying current one.
Wanted Daisy Model 25 or Norica Commando in reasonable condition.