I take it the replacement controller was a newer model?
Contact LSI support. My experience is mostly with Adaptec and Areca, both of which can recover lost RAID setups.
There will be a way in the megaraid software to do this, and it may involve re-establishing the RAID WITHOUT building (skip initialise).
If it works, Windows will see it’s NTFS partition again, although you may need to import the RAID in disk administrator.
Again, email LSI and check with them for the correct procedure.