Unable to start Oracle database
What you can try is the following while your database is in the MOUNTED estate

alter tablespace USERS offline;


and the same for the application tablespace you lost.

recover datafile 1;
alter database open;


NOTE:

If there is no backup and no way to restoring the lost datafiles, what you can do is backup any other important datafile/tablespace and recreate the database. I think it will be the less painful way to get a fully working database.