Rene Richarz has written a great guide on installing RetroPie on top of pi-topOS on your pi-top or pi-topCEED. He goes through removing part of the pi-topOS and then installing RetroPie, giving all the necessary command line instructions to set-up for the RetroPie instructions, which are on a different site. He even tells you where to go to set-up a pi-topPULSE HAT as a battery indicator.

You can read the guide here.

If you’re looking to buy a pi-top or CEED, you can find them on the pi-top website or from The Pi Hut.