I have a trailgear orbital valve that we used for mockup on a friends K5 but turned out to be too fast for what he plans on using it for. It's a 160cc(9.8ci) with an open center. Only has a couple trips around the block on it before he decided it was too fast. $300.00 plus shipping OBO or might trade for a 100cc or so unit but must be load reactive.
https://www.trail-gear.com/TG/Orbitals_Steering_Control_Valve_-_100__6_1_/i_0_0_606/_130043-1-KIT.aspx#.VXTwulL3anM
https://www.trail-gear.com/TG/Orbitals_Steering_Control_Valve_-_100__6_1_/i_0_0_606/_130043-1-KIT.aspx#.VXTwulL3anM