I inherited this bike, and immediately scrapped the Selecta Crankset. Besides sounding like an eighties hip-hop mc, it uses an unusual splined bottom bracket made long before shimano had mastered this technology. Instead of searching for the esoteric tool, I sawed into the cups with a reciprocating saw, nudged them loose with a hammer and heavy duty scew driver, and backed them out with a large plumbing wrench. Not exactly orthodox, i know, but the likelihood of finding the tool and appropriate replacement parts is pretty slim in my opinion.
I replaced it with a comparable campagnolo crank.

The other part you will likely have trouble servicing or replacing is the cassette. Chip a tooth on this and you will be out looking for a six speed friction cassette where the smallest cog, also serves as the lockring. Another strange transition period of shimano's move from freewheels to cassettes.

Other than that, I love the silver surfer. It's a tough bike I would easily lend to a friend for a ride through the potholed streets of brooklyn, with a reasonably fast riding position for a touring bike, and narrow handlebars which are great for the city.