Rebushing the valve gear is possible, but if the holes in the linkages are worn away, there's no real way to be certain that they are worn equally in all directions, so drilling out the holes for the bushings may in fact exacerbate the problem by moving the centers ever so slightly farther apart (or closer together). I'm sure there's a way around this through careful clamping and measuring, but if you're going to go through all that trouble, it may be just as simple to build new gear that allows for bushings, or make multiple sets of gear. It may be, too, that there's just not enough material to support bushing the joint.
I have an old Roundhouse Dylan that suffered from ancient and worn out valve gear. I gave thought to rebushing the entire loco, but decided to just replace everything with new. Fortunately, you can do that with Roundhouse locos. They haven't changed the basic measurements of their valve gear since day 1. The axle diameters did change, so I had to enlarge the holes in the frame for the axle bushings. They were already bushed to begin with, so there was no wear to worry about. The locomotive still runs flawlessly after 18 years. Later, K
