Hello,
If this old backup is good enough for you you have to recreate the database and restore it. In order to drop the database use
dbcc dbrepair(database_name,dropdb)
and then recreate it, if you have not a script please run
sp_helpdb db_name
before dropping and post the result, I will send you back the create database command. After recreating the database you may restore your backup
If it is possible to find the missing file (from a system backup, maybe?) you may stop ASE, put the file in place and restart. It is not guaranteed that the database will finish automatic recovery successfully, maybe something extra will be required.
Probably what you have in mind is none of the above, but somehow truncate all the transaction log and recreate it, you may check this
http://www.petersap.nl/SybaseWiki/index.php?title=Corruption_of_syslogs_(dbcc_rebuild_log)
I am not sure if this may work in your case
Best regards,
Kimon