You can easily pull the side cover on the driver side to check. These are really tough transmissions so if it was sitting on top of the oil you should be able to see how much damage it did. I am guessing after that length of time you may be looking at a rebuild kit, bearings, gaskets, and if badly pitted or broken some internal shafts and gears. SM-420's are common so if it is really bad source a replacement. I have bought and sold good ones for $100-150, which will be significantly cheaper than a rebuild or even parts.
