I recently bought a 7-speed bicycle practically brand new it still has a little rubber nipples on the tires apparently it got backed into in the garage and it bent the rear rim sideways my question is can I go out and buy another just like single speed 20 inch bicycle and put the 7-speed cassette onto that rim?
Typically, no. Because the chainline and OLD (over locknut dimension) is different between 7 speed frames and single speed frames. You need to use special conversion tools to adapt these builds, and even then you can run into technical problems. Such as trying to use a threaded freewheel single speed on a 135mm frame.
Because this will most likely be the issue you face if you buy two different bikes, it's questionable if you'll be able to pull this off.
Single speed bike will likely have 110 rear OLD and 7 speed will likely have 124mm (for hook dropouts) and 130/135mm for straight dropouts.
Your best bet is to buy a new rear wheel because the wheelmaster ones are not crazy expensive.