Dependant on the kit but you usually get the piston seal, spring, guides and sometimes packing washers or bushes, also lube.
I've got Maccari kits fitted in a FWB124 and a Webley Longbow. They are as good as any other kit you will buy.
In both cases the kits were easy to fit, as I would expect from anyone else.
Dependant on the stregnth of the Pound/Dollar the Maccari kits can be very good value.