Cogs are much easier to clean when they are taken off the freewheel body. You obviously also have to take them off to replace worn ones. I have a freewheel from 1984 that I have kept going by replacing worn cogs.

Is your freewheel Shimano Hyperglide like in the attached image? If so, see http://www.sheldonbrown.com/mega7 where he lists a hammer and punch as suitable tools. A needle nose pliers might also work. Or you could try a bottom bracket lockring tool if you happen to have one.