To a lot of us a 520 of that vintage is a Grail bike. Need to put a build thread on that in the C&V forum.

You can buy CR18's in a 27 for under $30 a hoop on Amazon. I'm running 700c on my 420 and worked fine with the stock brakes (not Canti). Bet it would work on that as well. I'd even consider a more modern drive train if whats on it is worn badly. I'm running mid 90's 105sc hubs with 7 speed indexed bar ends, Deore LX derailleurs with the stock crank to keep the vintage look. Rides wonderful. I'm about to build a 84 Trek 890 (Urban bike on 26's with 1.5 tires) and plan to run it 9 speed bar end.