Hi,
You will have to use same version i.e. SEP 12.1, can not use SEP 12.1 RU3.
First, simply try to repair using add/remove programs. Go to the add/remove program, select SEPM and then click on repair.
If any file is missing or corrupt then in that case you will required original setup file.
In that case if you don't have setup file then log a web case with the support and ping me the case number.
I will try to make it available for you on FTP.
How to create a new case in MySupport
http://www.symantec.com/docs/TECH58873