They were neat little machines. I was still using one in 2001, running Linux.
The easiest option is to use a null modem cable between the two PCs serial ports. For a file transfer app, I recommend Filelink which came with DR-DOS and therefore will be on the install disks for OpenDOS which you can download for free these days.
You only need it on one PC, it can bootstrap itself over the null modem cable.
The other option is to get a 2.5" IDE to USB adapter. You can get ones that do SATA too and will be useful in other situations in the future.