There is a Red urathane wheel available on ebay but they are all too wide and will go right into your spokes on a motorized bike even though they list them for use on one. Also the bearings were a bad and wont turn free in the one I bought. Money wasted.
Yes as 2 door says, a skateboard wheel of 54mm high x 31 mm wide works perfect. Make sure you get Abec bearings also. you can get a set of 4 wheels for around 7 bucks with 2 abec bearings per wheel on ebay in any color you could want. Black is best as grease wont show. They run smooth as silk. I used a lathe for great results today and a drill and file before that for a painfully slow groove install that was ugly but did work but was unsightly. it takes about 10 min with a course file and drill method to get a shallow groove. Wear a dust mask as the powder you will make on the harder skate board wheel is pretty nasty stuff. Pedaling is also easier with a skate board wheel as the stock tensioner sags as bearings are only on one side causing the wheel to lock up and drag on the side of the chain.
Its also important to use a thin 8mm lock nut on the skateboard wheel install on one side so it has no pressure on the bearings it so it can turn easy.