In terms of mechanical quality and stiffness, the Swift can definitely do a century.
I did a very hilly century on my Swift last year. I had swapped out the cassette to get lower gearing, put on a nice pair of grips and bar-ends.
My feeling is that after 70 miles or so, the harsher ride of 20" wheels does start to add up. Also it's kind of hard to do on-bike stretches, since you can't really ride no-hands. I had a saddle that fit me well, though, so most of the issues I had were with my hands and neck.
(And after riding my road bike for most of the year, putting high-pressure slicks, and a less-than-ideal saddle on my Swift, I start feeling it after only 20 miles....)
If I was going to do another Swift century, I'd use Big Apple tires and consider a Brooks saddle as well.