Your best chance to correctly ID your FS80 is through a Stihl dealership: the serial numbers they used at the time were very hard to broke down for the non-initiated.
Carb kits for those FS powerheads are widely available, but often those Zama carburetors do not take respond well to rebuilds and it's better to replace them. No problem as Chinese clones of the original are widely available, cheap and work well.
Regarding saw cutters. They are usually not advised for brushcutters under 1kW in power due to laboring the engine for getting them up to speed. The FS80 is slightly under that power so you are probably better off either using an ordinary blade or a 40T carbide tipped circular saw, which is the smallest sized manufactured by Tsumura and is considerably lighter than the solid body junk manufactured in China.