well so long as the hard drive is in the usb enclosure i'm sure there would be no problem installing just the the stock OS, beit 8.5 or 7.x. you may have to reboot your imac in 9.1 (or 9.2.2 should work) and you should be able to jsut install the old OS to the performa drive from your imac

so the usb enclosure might be your best bet