Most people building a DIY electric skateboard are doing so because they want something different or better than what’s currently on the market, and have something specific in mind. With a few exceptions such as those designed specifically for this purpose, the motors used in builds are designed for large scale RC model cars or airplanes, which don’t come out of the box with features useful for mounting drive pulleys to their shafts. Tacon, NTM, and Turnigy make some very popular models and none of them come with notches or keyways.
Fortunately you don’t need them to. You can notch the shafts yourself for use with a pulley of your choosing as long as it has a set screw or two. It’s easy if you have a dremel and a cutting wheel, and it can be done in about 2 minutes. This video should tell you everything you need to know.
I would recommend that you use the reinforced cutting wheels on your Dremel over the smaller non-reinforced ones like i’m using in this video. You’ll have a much easier time with them because they last longer, don’t shatter easily, and have a wider cutting surface.
When creating the notch, make sure you don’t take it all the way to the end of the shaft. Leave a lip there so that the set screw or bolt can grab it in the event that it begins to back out and loosen. This will help prevent the loss of the pulley if it does become loose, and will probably save you a belt as well.