Vintagemotif and KittyKat bring up a good point. My technique is not a cure all and will not restore bad decals or make them look new again but will brighten them, they do need to be cleaned well with sewing machine oil(decals). Remember they have tarnished with age so keep this in mind when they don't seem as clean and bright as you think they should be.
After cleaning apply shellac over them to protect them. Some decals are so brittle and basically rotten from age there is nothing you can for them but to leave them as is. the basic head japan black can be cleaned up well and will shine again but don't expect to get all the crazying out, although it will smooth out a lot. Are goal is to make the machine clean and pretty without ruining the original patina of age. All my old age wrinkles can't be smoothed either.
This is not a miracle cure but it will make it look much better and in keeping with antique restoration techniques without devalueing the machine. Skip