I've bit the bullet and ordered a Como motor (https://www.technobotsonline.com/919...otor-50-1.html) with a hub and I'll be fashioning a couple of linkages from the leftover HDPE from the side panels to make the spear with (with sharp metal attachments to it as well).

To accommodate other weaponry later on in life, I'll mount the motor to a strengthening brace which is attached to the the front wedge and the side panels so that if I wanted another weapon in place of the spear, all I'd need to do is switch out the front lid for the other front lid that has the other weapon attached to it.