Dirt and "gunk" are the primary killers of switches working well, if the switch has been subject to high humidity and moisture for long periods of time (think unheated storage) it may have problems with oxidation as well.
Cleaning out the switch is usually by some form of flushing, iso-alcohol may do that well or an ultra-sonic bath with water based solvents (often citric based). After a thorough flushing and solvent based cleaning, you need to get lubricant back into the switch again, or it will wear out much sooner than you want and they you will HAVE to replace it. Lubricants for switches are a very fine oil that clings to the surfaces and does not evaporate easily. Almost all the spray manufacturers make a lubricant, LPS, Caig, EML, etc. They are not the same as the cleaners. Check the labels or web sites of the manufacturers for the correct names.
Some switches may be made of materials that will melt or be damaged by some solvents, always be wary. Test first. Better to replace one switch than a whole lot of them.
There are at least two switch boards for the routing switches, they have different form factor hole for mounting the switches, the older ones used either Dialco or Isostat switches (forget which) which are unobtainium now. The newer ones use the ALPS or similar type switch lead spacing. I thought the new ones used the PH series E-switch spacing, but I won't swear to it. Calipers and the spec sheet are your friends.
schadow switches are readily available (although the numbers in stock appear to be limited at the moment) But I'm not sure the standard schadow switch is the correct size.
Cheers,
Alan