Mysql MyIsam tables get corrupted
MyISAM indexes tend to get corrupted when the server crashes (eg, power failure) or is otherwise rudely terminated (kill -9).  You did the 'right' thing by running CHECK and REPAIR.

The datafile size is a bit worrisome; that is worse than the usual (just index issue).  The crash may have happened between "writing" and "flushing", since MyISAM is not careful about syncing.

Changing to InnoDB is the 'right' thing to do.  For each of your tables, do

ALTER TABLE table_name ENGINE=InnoDB;


Caution:  The disk footprint for your tables will double or triple; check that you have enough disk space.

InnoDB automatically recovers after power failure and virtually any other crash.