For filesystems with 1k blocksizes, a backup superblock can be found at block 8193; for filesystems with 2k blocksizes, at block 16384; and for 4k blocksizes, at block 32768.

OK, so all those numbers are worth a try for e2fsck -b number.

-n causes mke2fs to not actually create a filesystem, but display what it would do if it were to create a filesystem.

This is exactly what's needed, but didn't IIRC exist in the e2fsprogs 1.18 that car players are built with.

Peter