I'm not familiar with the cart. However, two questions/comments I have after looking at the picture:
1) Does either the seat or the axle move so that the cart can be balanced? When it comes to Mini carts, I won't buy one that doesn't have some means of adjusting the balance
2) You would need to add footman's loops. It appears that the shafts have nothing at all on them that will hold the hold-back straps in place so that the cart will not run up on the horse. If indeed the cart has no footman's loops, you can get some welded on if the shafts are steel, or you might be able to use metal screws.
edited to add: I just had a look at their site & I see the shafts are aluminum. So, I guess metal screws would have to be what you'd use, no welding.
I don't see any reference to shaft length?