Are you sure it removed the system account? I've never heard of the before. Could it be that the import changed the system password to whatever it was in the source database? This is more likely.
If you know the system password from the source database, try logging in with that password. If not, try logging in as sys and changing the system password. If that fails, try logging in as internal and doing the same.
Please post results. I'd be really surprised if the system account was truly deleted.
Yes - surprising I know, but doing an import as SYS of a full export of SYS from another DB will overwrite SYS's password (or so I believe from experience of a client sending me a dump and me doing a full import).
I've never felt like going back and confirming it :-) but the parameter "full=y" on system is a dangerous setting. I have also fallen prey to that one. Get the password for system on the imported database and you should have access back to your "sys" on your system. (Then look for all the cr*p that got imported) into sys on your database.