Generally speaking, you can use a longer track axle with a few spacers. The Bicycle Research chromoly or stainless axles are excellent (and about $11-15 apiece) and most bike shops or
www.biketoolsetc.com will have axle spacers in all kinds of widths. Your existing cones, locknuts, etc. by and large swap over without a problem. (Occasionally I can add spacers to an existing axle, but too often, especially with better hubs, there just isn't a lot of surplus length -- and weight -- left there.) With some current fixed gear hubs, the axles are nonstandard funky, either with ledges to support sealed bearings or with slightly different diameters (e.g., the cheap Suzue's). Note that you may need to put most of your spacers on the left side of the hub (and won't be able to flip a double-sided wheel at that point) because otherwise you'll bring the chainline inwards too far on the drive side.
Usually it's easier simply to reset your rear ends to 120 mm. Before you do so check your chain alignment again -- it may work a lot better or worse with one method or the other.