You know, thats not a bad looking cooker, If you wanting something different, instead of trying to rebuild it, you might want to just re sell it for more than you paid and put the money towards what it is you want.
But at the same time, Im looking at it thinking how easy it would be to add the firebox to the back and move the axle back to adjust the weight. If it were mine, I'd just add a firebox to the left side cooker, then thow two pipe burners into the right side cooker. Once the axle is pushed all the way to the rear of the trailer, there would be room for wood racks under the outer front of both tanks, installing the expanded metal under the trailer frame and adding a short railing above the frame. A good size firebox should even the weight good enough and that trailer looks plenty sturdy to handle it.
Or, Id find me another small trailer and turn it into two smokers I could sell for about $2000.00 a piece, have at the most another $1000.00 ($500 for trailer and $250 a piece to add fireboxes to each one) in them and double my money.
Any way you look at it, there is a lot you could do with this one, and only have a grand in it, you cant loose.