Your gonna lose productivity with that size bucket if you found one. Stick with the bucket you have. You would dig 2 graves with an 18 inch bucket by the time you got finished with one with a 39 inch bucket. The only thing you will be able to heap is topsoil with that bucket. Once you get to virgin fill, you wont be digging anything. You will be exceeding the weight and power of that machine for the bucket to cut through anything