Make a mold of the part using RTV (Room Temperature Vulcanizing) rubber... Make the mold box out of plastic (I use old credit-cards that I get in the mail all the time, or sheet plastic cut from garage sale type signs). Suspend the wheel horizontaly in the mold box after coating it with vaseline or baby oil and fill the box halfway with the RTV rubber and let it cure completely. Then, cut keys into the first half of the mold, coat the mold face and the other half of the wheel with the release agent again, then pour the second half and let cure. After that's done, break open the mold halves and remove the master.
Now cut a pouring hole and air escape hole into the mold halves. Put the halves back together after coating with release agent again, and secure them tightly sandwiched between two pieces of plastic sheet or wood, making sure you don't push them together so hard that they deform.
Mix the resin (I use either "Amazing" brand or Alumalite) per the instructions in a paper cup (NOT styrofoam, the resin will melt it), and slowly pour the resin into the pouring hole, allowing air to escape from the other one. You'll have to tap the mold gently sometimes, to get air bubbles out to the top, but keep going (don'tdawdle now, the resin will start setting fairly quickly) until the resin is at the top of bothe holes... Tap it some more to remove any trapped air, top off if necessary, and let the resin cure. It'll generate heat during this time... After the resin cures, break open the mold and remove the part... It might not be perfect the first time, they seldom are, but with practice, you'll get a rejection rate of about 1 outta 5 castings.
Another way is to do a single-side mold, press the wheel a skosh past half-way into the RTV. Then, after that sets, remove the wheel and press down the other side the same way into a second mold-box of RTV. Pour the resin into each half and let cure, then remove the castings. You have two wheel-halves that you can sand down to size and glue together with CA or epoxy...