W98 is DOS + GUI, it will handle USB if BIOS does it too.
Is the one machine a used space feng shui?
You can get a miniPC and strap it behind the screen.
Get it from leasing returns and you can have a pro spare with same cost.
Linux box can be anything, L(ight)ubuntu will run almost anywhere and is like any Ubuntu.
NICs can be USB dongles and all dongles can have their completely own settings.
One thing, windows network will require two ports, one in and one out, and you just can't secure that with SMB1.
Hmm, maybe your ISA box has so much free space that you can stuff 2 miniPCs, ethernet switch and a KVM switch in it.
Then you can have a production machine, secure server and office workstation all in one case.
Linux box can also be a Raspberry and miniPC might fit in empty drive slot, almost done.