Upgrade theme from 17.1 to 21.7.2

One of my client uses an old version of Betheme (17.1). His website has recently been hacked and he asked me to update what needed to be (Wordpress/Plugins/Themes). 

I bougth the last version of the theme as he couldn't find the original purchase code. I tried to switch from old to new but everything was broken. They use a child theme, but the previous developer may have made some changes in the original theme. I can compare files but first, I'd like to know if you think this major update is technically possible with your theme without loss ? 

Moreover do you think it's possible that the security flaw was in this old version of the theme ? (I found many added/updated files by the malware in betheme but not in the child theme). If yes, is there a simple update I can do by replacing or editing a file ? 

I can PM you an access to front/back-end if needed. 

Hope you can help me with this delicate task ! 

Thanks,

Ben