Do I need any gaskets when replacing this? Are their any "special" tools required?

The cylinder and piston you are looking at are not listed for your machine. This is the correct one you would need: https://www.lsengineers.co.uk/cylinder-with-pistion-genuine-stihl-no-4140-020-1204.html The other gasket you would need is this one: https://www.lsengineers.co.uk/cylinder-gasket-for-stihl-bg45-bg46-bg55-bg65-bg85-leaf-blower.html