I would add that in the /boot/cmdline.txt parameters, I changed the resolution to 768x768 and everything is now centered and looks correct....except, the top few lines of the [TAB] menu in Mame are cut off....
@mitu good point. So one can use rotation of 90 or 270 degrees BUT the width and height is not exchanged on my proposed setup. Thus my hint only works flawless for 180 degree screen rotate installations.
So here I am six months later pleased to report that I finally have got this screen working. Kedei have changed the download page to https://kedei.net/download.html On a separate computer I downloaded the driver as a zip file, which I copied to a USB drive. I then copied the file to /home/pi (in RetroPie) I then unzipped it. This gave me a new directory called LCD_driver. I then did a chmod -R 777 LCD_driver. Despite the page on ali saying do not use FBCP driver I used it with the ./LCD35_fbcp command. The screen now works. I hope this post helps anyone who is having difficulty with this screen.
In Raspi-Config, Select Advanced, Overscan and then Disable. That should allow your GUI to extend to the edges of your screen. if it's still not to your liking you'll need to change the Overscan value in Config.txt in the /boot partition on your MicroSD card.