What "modern" PC has ISA slots anyway? The last one I saw was a special-order P4 board (Soyo, I think).
You might be able to get it working, but observe that the I/O ports and IRQ defaults for the XT controller are likely to conflict with existing hardware. There may also be some substantial BIOS conflicts as well.
I was able to get an MFM controller working in a Pentium II once, but it conflicts with things - I think I had to disable the onboard IDE controller in the BIOS.