If it has an Apple serial card then you're all set. There is a command you can type in at the ] prompt that will accept input from the serial port.
Then you get a text file dump of the Apple program called ADT. When you send it over a null-modem cable it will call the monitor program on the Apple and enter itself into RAM. Then you can use ADT to transfer and write a disk image of an Apple terminal program such as Modem Manager.
There are step-by-step instructions on the web somewhere, let me know if you have trouble finding them.
