These little carts sure are handy! I got this one from the local home improvement store. The end of the handle is designed to be configured for pulling by hand or towing by lawn mower. The design is clever, but the execution is a little bit cheap. The advertised weight limit of the cart is 1200 pounds, which is rather hilarious, but the cart can safely carry a couple hundred pounds of something heavy like gravel or masonry. I found that the longest part of the handle was made out of some pretty thing steel. It broke off from the end farthest from the cart, just outside of the heat affected zone of the weld.
The good news is that the original round handle mounts into a slot on the cart that is just slightly wider than 1.5 inches. I had a scrap of 1.5" square steel tube around, so I made a new handle out of that. A smaller piece would work fine too, you'd just have to shim the sides with washers. If you don't have a scrap like that on hand, check at the local hardware or "tractor" store.
In this picture, the piece on top is the original handle, which is much less substantial than the replacement.
I drilled a 1/4" hole through one end, and put the original handle bolt through that hole. On the other end, I cut drilled a 1/2" hole about 3 inches from the end and parallel to the hole on the other end, and then used a reciprocating saw to make a 1/2" slot. Then I made another 1/2" hole perpendicular to the other holes, to allow for a towing pin.
The final step was to drill the large rivet out of the pivoting handle. This allowed me to remove the piece that used to be welded to the round tube handle and discard it. From there I can stick the old handle end onto the new square piece and still pull it around manually.