Hi Ajeya,
Check these:
Alert Configuration in PI 7.31
By: Michal Krawczyk Michal's PI tips: Component-Based Message Alerting
By: Aashish Sinha 's:
Customize E-Mail Body and Subject in Alerts in SAP PI 7.31 – Java Stack Only – Part 1 – ESR
Customize E-Mail Body and Subject in Alerts in SAP PI 7.31 – Java Stack Only – Part 2 – ID
Regards.