Do I just replace one sprocket?
Yes, just the sprocket on the hub. This is what you want, and as I said earlier, possibly a chain (1/8" chain, different from a derailleur bike).
Even if you don't feel confident yourself, anyone you know with reasonable mechanical knowledge should be able to fit a sprocket on an SA hub.
The alternative is a 40 tooth chainwheel but good luck finding one of those on a cottered crank.