Yes you can use an old chips if they can operate at 13 MHz clock rate. That's why I put the UART to socket - sometimes it's not easy to find a datasheet of some specific chip... 16c55x recommended because of FIFO buffer.
It should be also possible to use XTAL with lower frequency and change the serial port initialization to get the same baud rate.
BTW this is a list of PMON commands:
The help function is working also for every listed command.
Downloading from ethernet, ^C to abort
h The help command m modify memory
copy copy memory d display memory
fill fill memory search search memory
compare compare memory uncmp Uncompress gzip file
set display/set variable unset remove variable
hi display command history vers display version info
load load memory from hostport call call function
cp2fl Copy Ram to Flash nvInit no options
nvErase no options etdiag enet diag. test
ildiag iline diag. test rmdiag sdram diag. test
mpdiag mass production diag. test pcidiag pci interface diag. test
pmadiag pcmcia interface diag. test leddiag led gpio interface diag. test
usbdiag usb interface diag. test plpdiag parallel port interface diag. test