There are amp and pre-amp simulator VST plugins, some will work in Audacity on Windows …
If it’s already tubey & dirty enough , you may just need dynamic-range-compression to reduce the variation in volume.
Audacity has a built-in compressor … Audacity Manual
Other free compressor plug-ins are available, e.g.Chris’s Compressor.
Paul-L’s de-clicker plugin can be used to remove any conspicuous plectrum noises from a guitar track , for a smoother sound.