How can I supplement a certificate if I cannot have one reissued from an educational institution that has gone out of business?

Hello, I am in charge of mandatory legal training for about 30 people in a manufacturing company.
Last year, I conducted several mandatory legal training courses through an external training agency!
While checking materials in preparation for a recent inspection, I noticed that some employee completion certificate files wouldn't open.

I tried to get it reissued, but the company has gone out of business, so I can't contact them, and there is nowhere to request a replacement.
All that remains at the company are the training announcement emails, attendance lists, some login records, and payment data from that time; individual certificates of completion and result reports are missing here and there.

In this case, is it possible to prove that the training was completed using the remaining materials?
I am also wondering if it is better to just have employees whose completion status is ambiguous, even after checking the materials, receive retraining.
If we conduct the training again, should I keep the previously reviewed materials along with the supplementary training records?
If the accepted materials differ depending on the type of mandatory legal education, I would appreciate it if you could also let me know which ones I need to check!