You could buy a cheap second-hand PC off eBay for less than £100, install your existing PCI card in it (but make sure that if it is a full-height card, the PC is in a big enough box - a lot of the eBay PCs are "small form factor" which won't take a full-height card). Install Mach3 - you can still download it off the Internet. Copy across the existing licence file and settings from the old hard drive, and you are all set. Don't think about a USB/serial adapter - it probably would not work in this situation. All depends on whether you want to sell a working system or maybe keep it and use it, or just see the back of the whole thing with minimum effort.

Well, that's how I would do it, but I have to say that I'm better at IT than CNC...