Oracle 12c recovery without system file
OS backups of the data files will only be valid if the database was shutdown at the time the backup was made, otherwise they most likely will not work because they will not account for the state of any data that was cached being written to disk when the backup was made. The system tablespace in particular is problematic because it contains the entire data dictionary for the database. Without it, the other data files are largely useless.
All data files must be synced to the same system change number that is in the control files before you'll be able to open the database; if you don't have archived redo logs to reapply missing transactions then you'd have to restore all data files and control files from the same cold/offline backup and accept whatever data loss that implied.
I would advise opening a ticket with Oracle Support, but without an RMAN backup or archived redo log files I don't think you're going to have many options. Maybe a recent datapump export of the data that you could use to populate a new db instance?