I'm guessing after so long the trans fluid has worked its way past the rings and into the crankcase with the rest of the trans fluid. Auto trans fluid is very thin oil with lots of detergents in it, so it can protect like regular oil and will even remove some grunge. Enough of it should have stuck to the walls of the cylinders to protect everything.
If you do get the motor, you're going to have to drain the crankcase, pull the plugs, and very carefully turn over the motor by hand. If there is a lot of trans fluid in the cylinders, you don't want to hydrolock the motor by trying to compress oil. If there is a lot, it will come out the spark plug holes. Left over trans fluid will burn off when you first start it - make sure you start it outside, as burning trans fluid is nasty shit.
Assuming that all goes ok, I'd add regular oil and run (idle) it long enough to get up to operating temperature, and then dump that oil again. The trans fluid will thin out motor oil, so after running it for a bit to hopefully mix all the remaining trans fluid with your new motor oil, you should be able to get most all of the trans fluid out. Running it at idle should prevent excess wear from the thinned out motor oil.
Fresh oil again, and then you should be good to go. I imagine all the trans fluid did protect against rust, so assuming the motor was in good shape when it was "pickled" lol, it should be in good shape now. Good luck!