why can't you? these systems cost $850 for a Pentium III with 1GB of RAM. You could build a low cost and powerful machine with modern components and have it run a VM just fine. The only issue, IMO, would be finding a motherboard with parallel or serial ports?

as I said in another comment, I used to do this for a containment solutions company. i soldered the cables from their custom machine to serial, then built a robust tiny PC, at the time it ran Windows 7, and put their legacy application on VM running Windows 3.11/DOS.