I originally tried a variation of jsuvman's solution; I only had one broken wheel, and so I replaced it with several layers of shrink tubing, building it up to the right size, then shaving the edges. This was several years ago; it worked OK, but I stopped using the machine, as I was going to the gym regularly.
Right now, of course, I can't go to the gym. So I went back to the machine, and found I how had another broken wheel; started doing the same solution, then find two other wheels about to come off. So I decided to go back to jsuvman's original solution, but came here to refresh my memory, and found DJ's solution. I bought 8 drawer guide wheels; they didn't come with lock washers, but I had enough, although it was difficult to put the nuts back on, on the side of the footpad which has the belt mechanism. It seems to be working fine, now; loads ot thanks to DJ and The Doctor!
Gene, April 2020