The emperor is fully identifiable if you could give a clearer picture...you've got Constantius Gallus or Julian II here. Reverse is indeed soldier spearing horseman, the legend is FEL TEMP REPARATIO (Happy times are here again! (because we just brutally murdered this barbarian)).
These are usually struck on bronze, its odd to find Auricalcum examples (the roman name for brass). this probably happened when an earlier sestertius was melted and repurposed for more recent coinage. It wont command a premium, though.