Can someone share a link to a good video of an engine swap on a Spider Box with a GY6, please. Also, will any GY6 interchange with the one that comes on the Spider Box?

The engines will swap out but you may need to use the original output shaft and sprocket to get proper spacing to the axle sprocket. There is a difference between internal reverse and external reverse models st stay with external. The GY6 cames in a short and long case version for larger and smaller scooter wheels - You want the short one. Your best resource is the dealers listed in the review section.