Trike tricycle help
 

Hi
I have been given a adult 3 wheeler its a Pashley Picador with 20" wheels and a Sturmey 3 speed set up.
The wheels are in a very bad / poor state and I would like to replace them, can I put BMX wheels on this?
I have removed a very large nut from the hub and the rings and the wheel is still attached, it looks like the axle is threaded. How do I get the wheels off and what do I need to do to the BMX wheels to get them to fit? Thanks