Canvas will run $800 to $1000 as mentioned above.
Upgrading to five lug hubs and 13" wheels will run around $400. I just did this on my pop-up, it's a
job and takes around 20 Min per side. Axle flip is a $10 if you weld otherwise $10 plus whatever you get charged for welding. Welding aside the flip is a
job and takes about two hours.
Bear Creek also sells replacement seat covers IIRC. Just in case you want to lose the Brady bunch motif.
Unless there is a lot of water damage the unit would sell for at least $1000 around here and wouldn't last more than a week on CL.