You can drill the rivets out by putting a punch mark in the center of each head and then going at it. These are 1/8" shank rivets, so get you some good sharp #30 drill bits, which will drill out the original rivet and leave a slightly oversized hole. Alternatively, you can get a good sharp putty knife and insert it between the window frame and the skin and shear the rivets by hammering on it--you will still end up drilling on some of the sheared shanks, in preparation for the new window, though.
If you have access to the back side of the rivets, then the best rivet to use to reinstall is a solid or "bucked" rivet with a 5/32' shaft. These are available from Vintage Trailer Supply with heads that will match the others on your trailer. If you can't access the back side of the rivet, then use Olympic rivets (also available from VTS), and you will have to shave the heads so they look like bucked rivets. Rivet shavers can be rented from some of the aircraft supply companies.
When you install the new window, liberally apply vulkem (Trempro 635) to the joint, and seal every rivet hole as you go as well. You will also need around 50 5/32" clecos, which you will use to clamp the new window in place as you drill the new mating holes. You will also use them while installing the "clamping bar" that joints the pano window to the center window frame.
This is a job that can be done in a few hours or a few days, depending on how skilled/exuberant you are, and how much trouble you run into getting the old window out.