Are you looking to display the picture ? One way could be to take a copy,use this as the display piece then store the original somewhere safe,then use photoshop to adjust the digital copy (tweak colours/contrast etc,fix cracks/marks,try to get rid of the yellow).Lots of factors can affect the colour in old photos (moisture,inks/paper used,uv damage etc),will ask at work on Tuesday if there is a specific cause (im a picture framer ) & what the best course of action is.
As far as the scanner,have you thought about trying to take a pic of it ? dont know if this can be done or not,but might be worth a try .