Replacement canvas will run around $800 I think, you can try Bear Creek for the canvas - they have a real good reputation in the pop-up camper world.
The real question is why has the canvas been damaged? If they packed it up wet then mold would happen very quickly and more of less destroy it, this would be the best situation. Most likely there has been water intrusion which will do a lot more damage than just the canvas. I would go look at the unit and inspect closely for water damage. Feel the floor and ceiling for soft spots and check in the cabinets as well. If the ceiling has leaked I would not spend more that $200.00 on it because you will have to rebuild the whole ceiling and that is not a small job.
All in all it doesn't seem like a bad little camper but there are a ton of them out there so take your time and you will find a great deal, especially this time of year.
Check out these 2 sites for more info. than you can imagine.
PopUp Explorer - The ultimate pop up camper web site
PopUpPortal.com - The Definitive Source For Pop Up Camper Owners | Home
I am racedirt1 on both of those sites and am also a moderator on pop-up explorer site. Frank Drebin is my moderator name if I can be of any further assistance.