Basically everything in .node-red with the exception of the node_modules directory or directories.
However there could be static content in some other directory (check settings.js for httpStatic)
Also check that the new Pi has access to network services such as databases, mqtt brokers.
Once everything is on the new Pi, you can rebuild node_modules by:
cd .node-red
npm install
My understanding is that this will install all required none-core nodes. I have never verified this!
You can find various discussions in the forum about backup and restore. For example https://discourse.nodered.org/t/backup-and-restore-node-red/86856/14
