From the picture it looks like a 4 1/2 gallon tank and the hard part will be getting the replacement tank. I had one like it and had to go to a commercial propane dealer to find one. This size is not readily available thru blue rhino type propane sales. It has been quite a few years ago but As I recall they were more expensive then the 20 pound barbeque grill type. The commercial supplier took my old tank and disposed of it no charge. Yours does not look real bad from the picture so unless the bottom is in bad condition I would use it up and then get a new one or I am sure a dealer would take partly full or empty. If I recall correctly the new tank was $50 dollars. Considering the cost to re-certify and not sure I would want to re-certify a 21 year old tank, I don't think it would be cost effective to update such a old tank so I would buy a new one.