http://sheldonbrown.com/bbsize.html
If you wanted to run cranks\spyder with removeable rings, this chart can help you mate -say a used crankset to the current bb.
Usually you would HAVE the new cranks and use the chart to select the correct axle length or that's what I did.
Determine the axle length, what ring amount, then copy the possible cranksets compatable with your current BB axle.?
An idea anyway.
I like old frames but prefer newer drive components.
We just removed a loose ball bb and cottered crank system off a Peugeot PX-10 and replaced it with a cartridge bb, Shimano 600 crankset and new rings.
-What a difference, my friend weighs 200+ and now he has a drive that is totally solid.
The new parts were 2 rings and the cartridge bb, the cranks were salvaged.
The rings were the most costly as we bought in a lbs instead of online.
This is perhaps only something to consider if the bike is quality enought to spend on an upgraded crankset.
Or bike that is intended to run many miles\chains -this is a good idea as the single rings can be replaced easily as they wear.