Hi David,
I for a fact don't know of anything that will soften a plastic bellows. You could try extending it fully & let it set for a day or two/three & see if that improves it. I vaguely recall something at the car parts joint that is a plastic conditioner for vinyl interiors. Wouldn't hurt to look.
I think you would save a slug or time & energy just getting a piano hinge & not bother trying to find an original. Being hardheaded I'd try to straighten the one you have. Second possibilty could be look for a small jewelry box at a thrift store & take the hinge from that.