I'm working on one now. It's a total design change and rebuild of a walk behind which was destroyed in a fire. Only the stamped chassis and cutter head itself survived. It was a decent unit as a walk behind, though a little underpowered, so now I have more power!
I'll reuse the cutter head with new bearings and bolted to the old chassis. The drive will now be the 540 PTO, with gearbox removed from my rebuilt snowblower reversed, so as to speed up the PTO, then a right angle gearbox to a drive pulley, and then to the three small drive belts of the original design, which I presume absorb drive shocks when you hit a rock. I've designed a new three point mount for the chassis so the cutter head can be hydraulically swung left and right, with manual up and down (I don't have two hydraulic functions out the back). The three point frame will sit on the ground for stability, and I'll slowly back into the stump as I sweep left and right. I'll complete it in the next few months.