QSL Andy seems half off 'G' land is into CNC these days and it's been over 15 months since i used mine.

As an idea could you not mount the nut into the base of the knee and use a belt to turn the screw in the configuration you have now? a bearing block could be placed under the plate you have the motor mounted to. a couple of pneumatic rams like used on hatchbacks would help with the weight if you use them upside down and a light spring over the ram section?