Lisa Sharpe has created a tutorial for making a marathon carriage. She’s put up 2 of the 3 videos and you can purchase the pattern from her also. I’ve purchased the pattern and I’m really looking forward to making a couple of these in different sizes.
Part 1 is above, part 2 is here: https://youtu.be/Zk8IrVzu77w?si=8kXQ4HI0eYRBECJz
These would not be for marathon carriages, but Lisa Sharpe has also found a vendor to make bike-like wheels in a size suitable for arena vehicles. You can find them here: Miniature bike wheels