Some loader valves have optional joystick "kits" - Prince for one. My old Case backhoe has a joystick, it's a very convoluted system of ball joints, pivot tubes, etc, works great but I wouldn't wanna have to build one from scratch - altho it WOULD adapt to two adjacent "normal" spools, that's what it's on right now.
I looked at all the pix I took while rebuilding mine (excess slop due to wear in bushings) but they're not clear enough to build from, only to re-assemble without messing up.
It would help if we had a brand/model # of your valve - and a good-sixed pic or two, preferably with any casting marks, etc... Steve