To reverse it all, you would need to remove/desolder those RAM sockets, solder in proper DRAM chips, remove the mux board, and put in the proper 64k 342-022x-x ROMs. I had to do the same thing to a 128k board I had.

If you have a EPROM burner, you could probably ask someone for a ROM dump of the 512k's 64k ROMs.

EDIT - almost forgot - you'll also need a 400k drive and ribbon cable if you have an 800k drive with a yellow-striped cable.