I don't know about the 115's, but that is going to be a gorgeous machine when it's cleaned. Except for that one small place on the back of the bed and the "Singer" on the back of the arm, you might be able get back all the color of the original decals. How fabulous! :)
It looks like you should be able to replace the face plate & tensioner with a decorated one from the 30's. Do you have another 15 that you could try it with to see if it works?
These old tensioners are really good ones, though - a couple of my treadles have them and once you get the two disks polished, they work beautifully. :)