Try the tape lift, dry cleaning pad and kneadable eraser first. If that doesn't work "Fels Naptha" is a laundry soap in bar form that we use to get stains out of my son's baseball pants. IDK, it might work. If it can't get any worse then anything is worth a try. If it's been seperated from the art and outer frame I'd have nothing against spraying it with lukewarm water to soak the fabric and to rinse it after scrubbing with soap. Use your air nozzle to blow dry as much as you can, not a hair drier, then air dry thoroughly. After that you still have the option of recovering it with fabric or just replacing it with precovered liner.