Welcome to the forum. So what kind of contaminants resulted while sitting for so long? If the fluid is burned, it burned before sitting, so the best thing to do is change the fluid and determine the condition of the internals when doing so, check the amount of junk sitting on the bottom of the pan. There will be a small amount from normal wear, but large piles of material is a bad sign. At this point, if the junk on the bottom is not large, since she is apart, pull the valve body off, disassemble and clean it, trans fluid lubricate during assembly, reset the two bands using the factory settings and an inch-pound torque wrench and see what happens. Also, large chunks of clutch material in the pan is an indication of possible failed seals and thus slipping clutch packs or bands that have damaged them, requiring a rebuild of the transmission itself.