Recreating the whole database is the only option here. Usualy in system TS there are only SYS's objects, and you can not possibly export those. So the only option is to:

1. do a full export (not because of the SYSTEM tablespace's objects, but because of all the other objects),
2. drop the whole database,
3. freshly create the new database with appropriate system's datafile
4. perform a full import to restore non-system TS objects