Does this fit a Stihl FS131/R and do you stock brush cutter harnesses?

Yes, It does look like this is compatible with your model. We also do have the harness in the links provided. 4119 710 9001 - Harness https://www.lsengineers.co.uk/catalog/product/view/id/16412 4119 710 9012 - Harness https://www.lsengineers.co.uk/catalog/product/view/id/22767