Do I need to buy the whole muffler? I want to clean the spark arrestor but it seems to be part of the integral muffler and no way to get at it (the 9 digit no. is: 164530911)

I have had a look, and according to the diagram for your machine you cannot get the spark arrestor separately anymore. I believe these did come on their own at one point but the design of them has changed. The correct replacement muffler for your version of the machine is: https://www.lsengineers.co.uk/muffler-exhaust-for-stihl-fs55-fs55c-4140-140-0617-replaces-4140-140-0609.html