About all you can do is drain the tank. Then drain the filter housing, then refill the tank with diesel and manually refill the filter housing. Start it up and let it run a couple of minutes, then repeat the drain and filling of the filter housing until all the no lead is out of the tank line( will take two or three times). Whatever ends up in the tank from the return line won't hurt much.
The 20 gal of unleaded is pretty much a loss if there's over two or three gal of diesel in it, it'll make whatever you dump it in smoke pretty good and run rough