With them being grooved it looks like you’ll have to scratch-build them possibly by turning them out of styrene rod. That is unless you can find a pulley for a belt drive that is small enough for your needs.
Tichy makes some really nice spoked wheels for their handcar kits (third item down on the link). For your Burro, I’d punch blanks from .010" styrene to form the second flanges, then install the wheels with the blank flange on the inboard sides.
However, if you don’t require six handcars and six trailers (one without wheels) [(-D], I’d simply make new wheels punched from sheet styrene - .010" for the flanges and the treads from a thickness appropriate to the rail size which you’re using on the flatcar.
Nautical models have a variety of sheaves which would make suitable starting points for a kitbash.
That’s where I intend to look for something to convert into 3.5mm wheels for my logger’s rolling stock. On the Kiso Rintetsu (762mm/2’6" gauge), the only larger wheels were on locomotives…
EDIT: After looking at prices, I think I’ll go with turning 4mm styrene rod with a vice-mounted drill. ±$5,00 per wheel would get expensive in a hurry!
I appreciate the responses and ideas. Definitely helps me think of different ways to tackle this.
Thought some might like to see my Burro crane flat in progress. Lots left to do, but it should look great once done. Working on a similar project for my O scale layout as well.
By the looks of the wheels you would still have to fabricate the inner flange.
If the HO wheels are way too big then there are a couple of wheels offered in N scale. Check out the Trainworx listings. They are dual wheel moldings but it would be easy to cut them apart. The biggest problem might be if the tires and wheels are a single casting. If so then you would have to turn them down to remove the tires. I guess its a question of how much detail you want in the bogie wheels:
I was thinking about this last nite,how about N-Scale boxcar wheels,or maybe even Z-Scale,might be able to trim them to your needs…Plastic ones…A Dayton Prototype truck wheel would be perfect for your needs,if you could find one to scale…TomikawaTT’s idea sounds feasible,also,with one of those wide cut-off disc’s,in the drill,fixed in a vise,with plastic rod held in your hand rotating the rod…or even a wood dowel,instead of the plastic rod…Good Luck!!