I’ve made an offline rosbag to rosbag2 converter that supports all default messages in ROS1 and ROS2. And for custom messages, you can use ros1_bridge’s instructions to create the mappings.
You can follow instructions at GitHub - leo-drive/ros1_bridge: ROS 2 package that provides bidirectional communication between ROS 1 and ROS 2 for the conversion.
But for summary there are 2 workspaces.
I hope this helps you.
If you have any troubles running it don’t hesitate to open up issues.