Welcome to the PUX Devin!
I would think that your odds of finding a serviceable used canvas for this Viking would be in the "winning the lottery" range. The good news is that if
the Popup is in very good
condition other than the canvas you could probably get a new canvas from Bear Creek (my choice!) or Canvas Replacements (also highly recommended by many here) for the $600-800 range allowing you to probably (at least) double your money in the spring on Craigslist or EBAY
If the camper turns out not
to be in pristine condition you might try parting it out online (EBAY) in the spring. You might be surprised what a working Popup faucet, or furnace, or 120v-12v converter box will fetch. The hollowed out "utility" trailer you are left with will certainly bring more than your $50 investment.
Either way you should be fine, money-wise