Anything that fits in the hole would be fine - its just to stop the two negs sliding onto each other. I could probably send you two pins as I dont think I will ever use 4x5 and I think I have 4, but a toothpick would probably be fine.
The red pegs are vital for keeping the sheets apart. If you use wood for pegs I think they should be discarded each time because of chemical absorbsion. It’s worth doing a search here on the subject of using these trays for film development, because much has been written about them in the past.