Steel frames with approximately 1" diameter tubing are the easiest to build.
Stretch frames and many chopped frames will require some fabrication for motor installation.
Fat rear tires (over 2") may require fabricating offset motor mounts.
I would suggest that you start with a steel beach cruiser frame and get the
motor/drive train mounted. Then you can personalize it with the saddle, handlebars, fork, etc that you like. Good luck.