These large focal plane shutters usually use a different material to modern small shutters, it's to do with dimensional stability (stretch etc). I supply material for large Graflex shutters to a US repairman and I know he tested and tried many materials first.

I'd try re-light-proofing the original curtains first, I've done that with a Speed Graphic and it's still fine 6 or 7 years later but a lot depends on the condition of the original curtains.

Not all these older shutters work on quite the same principles, Graflex use a long shutter curtain and a variety of slits of different slit widths and different tensions so the speed is dependant on the combination of slit width & tension, some shutters use elaborate mechanisms to vary the width of slit between two curtains that effectively get used as one (in use) while the best use two curtains, a Leica/Zorki uses a scaled down version.

The best of the old FP shutters in early Reflex cameras are modular (Thornton Pickard Ruby Reflex) and can be removed complete from the camera, I've never looked at a Miroflex so havae no idea how easy they would be to work on.

If you need any advice PM me. I've a backlog of large shutters (inc 5 FP) I'll be restoring over the next few weeks and some spare materials.

Ian