The copy command will already transfer missing blobs between repositories (it may just be necessary to force it to copy the snapshots again), but that only works if the chunker parameters are identical. Besides that there is always the option to restore data from one repository and back it up to the other one. That will also add missing blobs again. So the repair steps would to remove the damaged blobs first and then find a replacement somehow.
The erasure coding issue is Feature: Add redundancy for data stored in the repository · Issue #256 · restic/restic · GitHub . Although, as shown by Study of repository damage types this won’t be able to catch all types of damages.