For the headband replacement,
Beyerdynamic has replacement pads for their headphones that might work. Or maybe
this one? They have others, too, but the ones I linked look nicest (to me) and seem like they'd be simplest to try. Don't know if the DT 770/880/990 have the same sized headband, but my DT 1990 is just about the same size as the PM-3.
I was also looking at Reddit, and people were recommending fixes like bicycle/tennis racket grip tape,
iron-on patch cloth, and even heatshrink fabric tubing.