You will probably not be able to find a "kit". But if you look under your unit you may find a mounting bracket welded to your frame. Then the axle bolted to that bracket.
A welding shop or other steel fab shop may be able to help you. What would be done is with the trailer supported the axles would be removed by removing the mounting bolts. Then the mounting brackets removed from the frame. Then your choice of 2x2 or 2x4 boxed steel would be welded to your frame. Then the brackets would be welded to the box steel and the axles would be reattached.
Or if your axle may be just bolted to your frame Then your choice of 2x2 or 2x4 boxed steel would be bolted to your frame. Then your axle bolted to the box steel. Use the type of bolt and torque recommended by Dexter.
Here is some basic info from Dexter, See the installation sheet for details.