If you have complete courses with grades up to date, then, yes, it would make sense to import the courses with the users and the grade history from your 1.9 instance. However, it sounds like you now maybe have some grades in the 1.9 instance and some in the 2.3 or maybe I am confused?! Did you recreate the course in 2.3 from scratch or did you import it from 1.9? Did you bring the users across that way?
If you can, I recommend exporting the 1.9 course with users and grades selected in the export process and then using the course import to import the entire thing into 2.3. I haven't actually done this from 1.9 to 2.3 before but it is my understanding that you can bring across courses to 2.3 now with little trouble.
If for some reason this doesn't work, there are sql scripts that allow you to set all grades to not overridden so that might solve your empty quiz grades.