That's really easy....get some wood of the same species, cut to the shape you want, leaving some overlap, 1/16 to 1/8" thick, epoxy in place, and feather/blend in to match the riser shape. You will have to scuff the existing surface to get the epoxy to grip, hopefully all the way to the wood level.
The harder part is finishing the riser so all of the finish blends in, since I'm assuming the riser has already been sanded and has finish on it?
Or, get a contrasting wood in 1/2 dowel, fit the plugs, and tap in place until flush. If these are permanent, tap in with tiny amount of epoxy on the leading edge as they enter the riser, so they don't come out, and water doesn't go in.