The paths to SQL Server Agent subsystem DLL’s are stored in table msdb.dbo.syssubsystems, hence when you do a restore of MSDB database you have to follow the below process to delete the entry and repopulate the same:
use msdb
go
delete from msdb.dbo.syssubsystems
exec msdb.dbo.sp_verify_subsystems 1
go
You would require restarting the Agent service to be back in business.