There should be holes on the back of the motor that will have it mount like the smaller motor they showed, not like the emax 2215 c pack motor, the bigger one. Your radial motor, the new version, will work great for the explorer, but if you want to use the old version where they mounted the motor behind the firewall, here’s a link to that exact motor. They are about $20 and will work with all the electronics from your radial c pack.
https://store.flitetest.com/emax-gt2215-09-1180kv-brushless-motor-emx-mt-0409/p694266
I hope this helps. Just to state it again, you have the newest version of the c pack which they don’t show in the build video yet. The old c pack motor that I linked will mount like the one in the video. Your radial motor will mount very similar to the smaller motor they showed, you’ll just have to screw the screws in from the back of the firewall. Good luck!