My sister's Compact Jr, loaded and ready to go, weighs about 1700 pounds--so a 2000 pound axle
ought to suffice.
Her popup had a sag in the middle, so I bolted in some support arches made from electrical
conduit. To get the right arch to the roof, I supported it upside down along the long edges and put bricks inside until I had a nice curve, then bent and cut the conduit to fit. Worked pretty well.
If you're going to build a new frame, you ought to think about a mount for a spare tire. Hers didn't have one; the tire was just sitting inside the trailer.
Every winter I work on her trailer, fixing mishaps and making little changes that she requests. I've posted a couple threads here about those winter rework sessions:
Compact Jr. Winter rework session
Sis' Compact Jr Rework Fall 08
Sis' Compact Jr. winter rework/repair 2011