Definitely Tom, but a quite bit more involved project than I want to take on & rocks will always be a problem.
I cut up a wooden handled wire brush, the flexible wire bent in a U & stapled into the wood... just shoved the U profile in the hole of the drum, then a 3mm steel pin & locked in place with the set screw. Switched the drums left right so it drags the wire bristles... works muuuuch better!
But the motor is still lacking in power... it'd be good if I could get a stronger brushed motor (more poles would that be?) that would fit this spur gearhead so I wouldn't have to completely rebuild the mount. I found on RC4WD the inner shafts, so shortening the drums is possible... try half the width & double up the holes/bristles for a smoother continuous contact.
Here's a vid, grinder cutting away at the sand layer below the cement...
https://www.youtube.com/watch?v=3iVrRdD9nMc