The VA will replace Civil War headstones, if the original is broken or illegible, or the inscription is wrong. All you have to pay for is placing the new headstone (or you can get some friends together and do it yourself). I suggest contacting a local SUV camp, or Civil War Roundtable (or even the SCV - my SCV camp has set markers for Union veterans as well), or a VFW or American Legion - they can probably help with the paperwork for the VA.
Good luck!
Geoff Walden