Now,... the problem of getting the new one back on, you can either send the shock to a shop to have it disassembled, or.....purchase a new nut and try this method; thread the pinch bolt into the nut backwards, forcing the spanner nut open enough to get it over the shock body, but you will take the chance of breaking the new nut.
RipnRol actually made a two piece spanner nut with two pinch bolts, do a search on his thread.