Double check to make sure it's stainless and not aluminum. A LOT of that style keg are in fact aluminum. IF it is stainless, then you could seal up the side bung, and stand it up (you'll need to adapt it to install an airlock/blowoff tube in one end). You could weld in a 4" TC ferrule into either end, and then adapt a 4" TC cap to hold a liquid post (with dip tube), a gas post, and thermowell. That way you can ferment, get temperature readings from the middle of the batch, and then push the finished beer out with CO2 to carbonate/serve.
I did that to a 50L sanke keg (for ~10 gallon batches).
Or you can just keep it and use it for cask ales.
Personally, I'm using sanke kegs for fermenters. I KNOW they're stainless steel and it's easy to adapt them to being fermenters.