I would not rebuild. I would mke sure the top is flat and square and put a shim under the bottom — perhaps screw a washer or two on the short corner so it doesn’t wobble or use one of the chair glides that come with a short nail attached. Or if all else fails, get out your sander and work on the long corner until that puppy sits flat!
I used a kitchen base cabinet that I rescued from Habitat for Humanity’s store in Raleigh, NC. I added a 2” x 4” brace across the middle of the top (probably overkill), constructed a top of 2 layers of 1/2 inch plywood glued together and glued to the top of the cabinet and the 2x4. Then I sanded, primed, and painted the whole thing with brown Rustoleum paint. It looks great and has plenty of strength.
My 75 gal sits on a store-bought fish tank cabinet that has 1x4 corner pieces and 1/4 inch pressed wood panels. I thought that the construction seemed flimsy, but it has held the weight of a 75 gallon tank for a long, long time. Of course it was designed by an engineer. For engineers, 2x4s are overkill. But if you’re uncomfortable going lighter and you’re not an engineer, then go for it with 2x4s and be happy. I’d love to see a photo when you’re finished. Good luck!