I believe you can use an AT PSU from an old PC in an A2000. It might take a bit of soldering and time, but other than that it should work. You should be able to find something about that by googling Amiga AT PSU or something similar.
You should be able to fix the mobo, but again it may require soldering and time - theoretically I assume you could use the 68000 CPU socket from an A500, but I'm not sure on this, and someone may want to correct me here.
Hope you get it going again soon :-)