Get yourself some solid copper wire, like that used for household wiring [Romex] of 14~18 gauge. Strip the insulation off of one strand or just use the ground wire, cut it at a long angle with a pair of diagonal cutters. Use the fresh cut end as your 'pick' or tool along with some Kleen Bore Formulae 3 to remove the rust. If it gets worn too smooth or rounded, just nip a small increment from its end to 'resharpen'. The Formulae 3 will penetrate wonderfully and loosen the rust as well as prevent future rusting and the copper 'tool' will not scratch a thing nor harm the engraving. It will be tedious work, but you should only need do it once. Do not be afraid to bear down or be agressive with the wire's end, you cannot hurt anything, metal wise. You could put a gouge in the wood though if you slip so pay attention. Use a piece of wire long enough to be comfortable held in your hand and bend the last 1/2 " 90 degrees or down for the working end if it makes it easier for you to use it that way.
For the best thing to re-oil or wipe down engraved surfaces, get a piece of real lamb's wool leather that has been clipped close. There is simply nothing more effective to insure you get a coat of oil on and into the engraving. A small piece will last you a lifetime. I have one that is roughly 4"x6" that has been in use for 40+ years and if anything it is better now than when first put in service.
Hope something here helps you out.
