I bought some lead, I just have to make the insert. I finally got to weigh the kart since it is new to me and not thought about weighing it since I have only done locals in it. We found it to be quite heavy it was 220lbs with not much fuel in it. So I think we will be fine with what I have.
How did you construct the insert? I was thinking about forming the lead into small octagons and then tying them all together with solid steel wire, then making a foam padded cover for it.