I've bought a few parts from Brooks for when I'm rebuilding forks or shocks and £130 is a good price if it includes Seals, renewing the oil and re-gassing
I'm guessing it is a Sachs shock as satndard, a little tricky to get apart but easy enough when you know what your doing and have the right tools
alternatives would be something like a shock from a newer Ducati such as an ST3 or ST4, or even a 620/800/1000 Monster as they are all 305mm centre to centre on the mounting holes. If going for a Monster one I would see about swapping the sping off your one to the replacement as I think it is a bit stiffer.
Good alternative would be a Nitron R1 shock, although a Monotube without an external reservoir they do contain a floating piston that seperates the oil from the gas and hydraulic preload is an option
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