For keeping bikes eligible for retro events - which is pretty much the same as making sure they work with 5 and 6-speed freewheels - I like to keep an eye out for 86 BCD cranks.
They make nice super compact doubles. Examples from Stronglight and Sakae Ringyo do come up for sale regularly, and with a bit of luck for very fair prices. New rings are available from Spa Cycles, if needed.
Currently operational on bikes in the non-fixie stable: