CMS Made Simple Forums

A friend has CMSMS 1.11.13 installed. After PHP upgraded to 7.3 the site is completely broken. Cannot show page (error 500, lots of php errors like this:
Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP;
ADOConnection has a deprecated constructor in /lib/adodb_lite/adodb.inc.php on line 184)

I've read that I should first upgrade to 1.12.1 but how do I do that without the site running?
I can make backups of the database and all files.

Regards
Peter

The best option would be to copy it to a server where PHP 5.x is still available. Upgrade it to 2.2.10, then copy it back.

New status: Site is up after modifying some php files. Yet I get a lot of deprecated messages. I can fix that.

What I want to know from you experts is how do I upgrade properly?
From CMSMS site:
In order to upgrade from 1.11.13 you should first upgrade to 1.12.1.
In order to do that you must first upgrade the modules.

Ok, on the Modules page there are some red marked modules that clearly states that the highest version supported is 1.11.9. Some of them have an option to uninstall, some have not.

How do I go on from here?

It depends on the module. Some have an easy migration path (e.g. ListIt > LISE), while others wile require more thought (e.g. AdvancedContent to CGContentUtilities).