Clean all parts.  The drum needs to be waxed, I use 3 coats plus a coat of mold release.  Take the pieces of the broken ring and treat the same.  Put the pieces on the winch and use tape to hold the parts in place on the winch drum.  Packing tape works well.  This will be a finished surface so use minimum tape to keep as smooth as possible.

Place the winch drum upside down on wax paper on a flat surface.  Using auto body filler (Bondo), completely cover the outer portion of the plastic ring and build the filler up on the winch drum.  You are creating a female mold of the plastic ring on the winch.  When the body filler hardens and while still green, dig out the plastic ring.  When the body filler cures clean up the mold and fair any imperfections.  Apply mold release to the inside of the new female mold which is still on the drum.  You may want to tape around the bondo and drum to prevent epoxy from running out.  This also ensures the mold stays in position on the drum.

Take the inner core, the part the winch handle goes into and put a layer or 2 of masking tape on the outside.  This is creating extra clearance necessary for this part to spin inside the new cap when the winch is reassembled.  Wax and treat with mold release.  Cover the top with packing tape as this piece will be completely covered when all cures.  Place this part into the winch drum.  This completes the mold.

I used unidirectional carbon fiber cloth for the new cap.  Separate strands and saturate with West System Epoxy.  I used 207 hardener for its ultraviolet protection or the cap will eventually yellow.  With the winch standing up. lay strands into the mold circling the winch overlapping strands.  Push strands down packing as tight as possible.  Build the strands up to the top of mold and slightly higher.  This will be the finished cap so you don't want any voids.  Finish by covering the winch with a layer of carbon fiber cloth.  This will give the winch a nice appearance when finished.  Cover this with plastic or Saran Wrap.  Put anything flat and smooth on top and weight down to compress and let cure.   

When cured the winch top is completely covered.  You'll need to cut a circle in the top of the winch just large enough to clear the winch core which needs to be removed.  A little pounding may be required to drive it out.  I used a Dremel tool to do the cutting.  Be careful not to breathe any of the carbon fiber dust.  It can be deadly in you lungs.  You can now chip off the Bondo on the winch drum.

Clean up and reassemble parts.   You should have a brand new winch looking identical to the original minus the Maxwell name.

Hope you saved your parts.  I still have a broken ring if anyone wants to try this project.