There are many many plugins for that application. Voxengo makes several plugins that work for this: CurveEQ, GlissEQ, etc. I guess there should be alot of free spectral analyzer plugins as well...
Be warned though: Getting close to a guitar sound by matching the spectrum is not all it takes. You get the overall balance close that way. But how the signal and frequencies behave over time is not represented by the spectrum at all and has a big influence as well...
