If you want to emulate dial-up, it would seem to me what
you'd really want is a driver that runs under Windows or
Linux over a serial port that would pretend to be a modem,
allowing the old machine to "dial" the way it always did.
The contemporary box would connect to the net in its own
way, but would seem to be a modem to the old box, even
though the data is tossed around on a VPN.
- John