Hi Annal,
Good to hear that it is on Shopify Plus. Can you please tell what scripts/events are you looking to add on the page. If you have access to checkout.liquid, please send me that as well. If you do not have access to checkout.liquid file in your theme code, then contact Shopify Plus Support to gain access.
One suggestion - It would have been easier to manage these kinds of scripts if you had used Google Tag Manager, as it is more customizable and most of the scripts could have been added on GTM instead of each code file.
Thanks,
Sajat