Does it have DOT markings on the top? What's the rated PSI? If it's rated for at least 1800 psi you should be fine.
I'm not familiar with the valve used on Nitrous Oxide...but you may be able to find a replacement valve that's CGA320...and be in business.
The shop that does Hydro testing here in South Florida...refurbs tanks and replaces valves. I'd check with your local shop to see what they quote to change the tank over, if that's even a player.