My bike came with a 5 speed 14-28 Suntour freewheel. Back in the day, I could have had that freewheel customized to get the desired effect, but that is no longer possible.
Rear dropout spacing is 120 (actually ~122) mm. Wheels are 27". Last summer, I spent 5 months touring Europe and Canada and carried a roll-up 27 inch tire because they are so hard to come by. I considered changing to 700C wheels, but the cantilever brakes have no adjustment for wheel height, so I would have to replace them.
As you can see, one thing leads to another and I am content just to improve the drive train a bit. The bike is tough and reliable, and fairly easy for me to maintain and service, and I've kinda bonded with it. Those are things that I might lose with a newer bike.
Honestly, I am not convinced that 9 or 10 speeds is either necessary or practical for touring. Chains are narrower, and therefore weaker, and not so easily found. Wheels are deeply dished and also weaker. I met folks touring with that kind of setup who were using just a few of the 27 gears they had available because the others were out of their comfort ranges, and carrying all the weight that those extra gears require.
My objective is to have as many gear ratios in my comfort range as possible, in a setup that is easy to repair and maintain.
Thanks for the advice on chainwheels, RussSeaton. I'm more confident now that I can get this bike into the kind of configuration that will suit my cycling style.