There are PCIe to PCI adapters, would that work for you? I have not used them before but there is one on its' way to me as I also want to keep using an older PCI card in a new computer. I can report back on that once I have installed and used it. It may take some extra work to make things fit from a physical point of view, perhaps I need an additional right angle adapter to mount my card sideways as height is limited in my case.
(mine is not for anything PLC related btw)