Antonio,
I cannot see your pictures (permission required it says.) However I do not think the diagonal tube you refer to is original. As '523 is one of the early Acecas I wonder if any overriders or their mounting brackets have survived ? It would be clearer were you to read the early overrider thread in this section.
I realise that you have bigger problems than the overriders, but I ask just in case something has survived. Good luck with the restoration
Barrie