Thanks for the files. Managed to copy cleantmp & codeset. Having problems with removing the invalid cron folder. I emptied its contents.

Problems:
1. Refusing to overwrite with cron file you sent.
2. Not sure why but when cd to it, it puts me into /dev/ttypdir. This is empty.
3. 'rm -r cron' fails with error 'rm: unable to remove directory cron: File exists (Error 17)'.

'integrity -e' now only complains of the cron folder.

Try
rmdir cron
If that complains that it is not empty,
cd cron
rm *
cd ..
and try again.
Also see if
ls -li cron
returns a link to another file, then try deleting it instead.

Is the system now running more or less normally?

1. 'rmdir cron' refuses with 'Directory not empty'
2. cd cron, rm * then tried again still refused with same message
3. 'li cron' return 'total 0' and doesnt return a link to another file.
4. 'find /' as root still brings errors (screen fulls) as before.
5. 'cd cron' then 'pwd' shows '/dev/ttypdir'. 'ls -al *' shows its empty.
6. Tried 'mv cron /run/.' & 'mv cron /run/cron' fails with target (/run/./cron & /run/cron) not a directory.

Not sure why i am redirected to this folder as the cron is not a link and properties are 'drw-r--r--' . What exactly is '/dev/ttypdir'??

The system is ok (Mon-Sat) but Sunday the /tmp got the huge crout file and fills up /root. It haapened again this last Sunday. I am sure it will happen again this Sunday since 'find /' not working.

I checked a 5.0.5 system, and it has no file/directory named ttypdir.
If you haven't created your own jobs, the only job that would be run on Sunday (on the system that I checked) is /etc/cleanup. Disable it in /usr/spool/cron/crontabs/root.
Then at least the system will not go down again. All that this job does is truncate some log files, and does a 'find' in order to remove any 'core' files more than 7 days old.
Have you tried removing 'cron' from within /var/opt/K/SCO/Unix/5.0.5Eb/etc/default, as opposed to /etc/default ?
It may be time to re-install the operating system.