HP requirements vary depending on what you are cutting. And if you are short on HP for the conditions you can just go slower, assuming you have a HST or extra gears.
there is a guy on eBay (dnequipmentparts) that sells rotary cutter kits, Rotary Cutter Parts, Finish Mower Parts items in DN Equipment Parts store on eBay!
(gear box, stump, jumper blades), the PTO shaft is not in the kit, or the deck or tail wheel (s). DN Equipment they also have a web site as well. and at one time a company made a cutter using a car tire to drive a disk (the gear box), called a side winder.