Check ebay or take this dead one to electronics workshop - maybe they can repair it. Maybe only few caps need to be replaced.
You can use temporarily 17" PSU (original or ALS one), but in this case you won't be able to close iMac's back panel. "Plug hole" in 20" does not align with plug socket of 17" PSU. That's why it's temporary solution (or if you'd be a desperate, you can cut new hole in back panel - been there twice 😉 - not the easiest job and visual effect isn't pretty - but who looks at back of iMac 😀).
