The hard part of anything like this is the manual labor to organize shit.
If it was my call, I would spin up a clean new system and using a report form the old system only import the valid assets.
Manually do the fucked up assets the right way. and then set a migration date.
Kill change access to the old system and use it only for reporting purposes.
Yeah, that does make the most sense. And the manual labor is the worst part. It always is.