So you want to use a 901 battery on the 501.
You need to make the 510 compatible for the 901.
Take a 510 battery, and use pipe cutters to cut the end off of it. Then take a 901 atomizer and strip it out for the thread. OR
buy that thread from Nerf.
You could use the empty 510 battery for the shaft.
Take the 901 atomizer female plug and solder it onto the 510 battery female end. Be sure to do this inside the tube you are using. Then push them both into the tube, and your done.