You don't seem to have any bites yet. You'd need an f big ballscrew to move the knee assembly up and down but that's what some machines do. Counterweights and/or gas springs will help to reduce the load.

That looks like a fine, heavy machine. It's a true universal machine with pivoting table but that's not easy to utilise in CNC although arguably there'd be less need to. Also appears to have the full set of power feeds and rapids, so can't help thinking you'd find a better donor, ideally an old CNC machine needing updated electronics. Having taken both routes, I'd recommend that way given the choice.

Andy Pugh has a very similar Harrison mill that he converted to LinuxCNC:


Some of the German / Swiss machines use this concept for all Z motion. If you follow SMProtary or This Old Tony on Youtube, you'll see what their Maho machines look like. At least you shouldn't get much backlash in Z!!

TOT:

SMProtary:


I also saw a Brit (John someone) who'd converted a Bridgeport to CNC and had an encoder on both the knee and the quill - got a suspicion he motorised the knee but don't recall his YT handle.