IMHO, by the time you buy rims, hubs, spokes and tires, you will have spent a lot more money than just buying the Simplex reproductions from Coker. To get the same size tire as a Simplex in a 21" motorcycle tire, you will have to find a 2.50-21. I looked for a street tire in that size and never found one. I used 2.75-21 on a Sportsman, but that tire is too large for a regular Simplex fender. Also, being that the only tire I found in that size is a reproduction of a Firestone from Coker, there is no money savings.
Now, if I were going to build a Simplex, and was trying to ride it over 50 mph, I would look for motorcycle tires and brakes. That said, I have looked around and have never found a tire that I thought was suitable. If you find something, please post it up so we can check it out. Thanks, Rick