stem 20.9mm : Again,keeping the original is the only alternative.
handle bar 23.4/24.9mm : It is possible to fit an MTB size handle bar with a 25.4mm clamp diameter.You will need to gently prize the clamp open with a screw driver to make it fit.Happily,on my bike the original bars are in quite good condition.
front axle 7.9mm : no way around it other than to keep the original hub.
wheels 550A (close to 22"): I had thoughts of purchasing some new rims and lacing them to the original hubs.This is not going to happen as 550A rims are not available and the brakes and fenders will not fit a new 24' rim. Luckily,my rims and hubs are in near new condition so I will just have my LBS relace them with some shiny new spokes.
all stripped down and shinnied up.
I polished all of the chromed parts using 'chemtec'. Just soak for a few minute,scrub with an old toothebrush and rinse. All shiny and new!