You can create KPI base on the Roles and responsibilities of a Site Manager
Example
Site Manager is responsible to ensure right amount of manpower is available at site or to complete the work is scheduled time.
Your KPI could be like log of actual manpower Vs Required showing as a % achievement or Manpower at Site week x%, Month y% like that.
Same way you can have
Scheduled dates vs completed or not completed this will give % activity completed on time.
Similarly you can have more KPI's base on the Roles and Responsibilities assigned to him.