Hi Jessica,
For replacement
windows go to your local glass shop. They can easily replace the plexiglass (acrylic) windows with ones custom cut to fit your frames.
You should search online for a
fridge, after measuring your space. Dometic made the original ones for the Burros. I believe that the one for the Burro 13 was Dometic 2111. Which is a 1.9 Cu ft. 3 way. It is no longer made but several models will fit into the space that you have. But your first decision is how to power the
fridge. The most popular are 2 way, which can be run from 110V AC (called "shore power") or from
propane. The
propane is important when you do not have an
electrical hookup. These fridges from a RV shop will cost you $900, but you can look for used ones. Your carpenter/mechanic "friend" can put it in for you.
Tires are best found at your local tire shop. 13" is not common but is still available.
Hubcaps you can find from "Mike's Hubcaps" find him on the web. This will require you to measure the distance between the cap nubs on the wheels, but they tell you how to do this. I have Mike's baby moons on my Burro.
Rick