With my Phillip's Twenty I re-tapped the bottom bracket (I have these taps available to me) and installed a 73 mm Shimano UN54 cartridge with a 115 mm spindle (if memory serves).
This took all of 30 minutes and the bike is now rocking a Stronglight crank with a 52 tooth ring and I have had no issues with it at all.
I gave up on trying to find a 7 series spindle for the 20.