You have nothing to lose by cleaning it up even if you only end up with a curio.
I was given an Albini Braendlin rifle circa approx 1855. http://www.militaryrifles.com/belgium/Albini.htm
It looked like a lump of rust with a lump of wood stuck to it.
It had been stood behind a shed for decades and the inside of the barrel was solid with rust. I had to chip it out with a sharp piece of bar. needless to say the bore was useless.
The back sight had been snapped off and the forend had a lump missing off the end and the muzzle barrel band was missing. The bar that connected to the hammer that hit the firing pin was snapped and the knob that was used to open the breech was snapped off.
I managed to free up the parts and get it cleaned up a bit and steamed the dents out of the butt. Now it looks like a gun that has had a hard life.
It was worth the trouble I think even though it is worth naff all.