Old game no longer works post Experimental mode removal how, and what, do I fix?

Filtering enabled means that now, the server has ownership over replication and the players have power only over their own character.

Localscripts are still able to: Change certain properties on the humanoid (Walkspeed, JumpPower will make you jump and walk differently but the server will see the values as their default number), Change the CFrame of their own character, Play Animations.

You will need to use RemoteEvents and RemoteFunctions in order to make the game work properly again.

Everything that needs to be replicated (everything that you want to be seen by every player) will need to be transferred to a server script and fired by a remote event if it is triggered by the player locally somehow.

I want you to please view these articles which will help you understand what you need to do: RemoteEvents and RemoteFunctions, Network Ownership, Converting from Experimental Mode, Game Security.