I just tied some Red Fox Squirrel Hair Nymphs very close Mr. Whitlock's design. He offered up options or alternatives in his pattern recipes (see link below). Which I used. Mr. Whitlock was a very accomplished tier and angler. He studied his craft. Which was catching fish and the best way to succeed at it. Hence some exceptional fly designs, equipment contributions, and angling technics. Here's a link discussing the Red Fox Nymph (https://davewhitlock.com/wp-content/uploads/2014/05/RFSHN-story-and-tying.pdf ). He offers up substitutions in that.
As a side note: Sometimes adhering to an original design is impractical if not outright illegal. I tie steelhead flies. Often trying to replicate the original design to the extent possible. Those old timers used some materials not available anymore or presently flat out illegal. Polar bear for one was widely used amongst other things. I do have some pre 1972 polar bear that I use in tying for my personal use. I keep the CITES certification and all. A while back on this forum there was a problem when some ties/materials crossed an international border. A customs agent decided to give it a looksie. Some of the materials had an international ban for use. It was done innocently but It put the person doing the shipping into a world of grief.
Having said all that, a swap that has replications using original materials and design may be a good one to contemplate. Those swap flies will end up in a display. Something to consider as I'm game. This one though, do the best we can.