After having a look at the database after an import, I believe I have a work-around for the non-searchable courses. The current IMS load creates a course without anything in the course summary field, and this is assigned the value NULL. However, when searching, a course with NULL as the summary will not be included in the search results. I'm not sure which end the problem lies, whether the search should be able to ignore the NULL, or the IMS load should not create NULL entries or whether the database structure should not be set as NULL by default. Anyway, if you put this line in your lib.php file, around line 354, you should have a suitable workaround.
$course->summary = '';