Replacement for A.O. Smith FSG 40 222?

[JJVBVA]

My 86yo mother was told she needed to replace her 17yo A.O. Smith FSG 40-222 (LNG) water heater because it is leaking. Was quoted $2K for a 50 gallon replacement. Not sure brand or whatever. Fact is she doesn't need the Cadillac. 40 gallon is fine. Only two people in the house. Does anyone know of models of a similar size that I could throw in there with minimal (ideally) no modifications? Not looking to spend a fortune, prefer something that is good for under $500 and readily available. Thanks.

[Bobice]

Check out : http://www.nextag.com/ao-smith-40-gallon-gas-water-heater/compare-html

[JJVBVA]

Thank you. I will be able to get exact specs off the existing one later today so I can match up the replacement better. I spent some time searching unsuccessfully for specs on the existing unit on the internet. I've not done one of these before but it looks pretty straightforward. Just trying to minimize making mods. Primarily would like to keep the exhaust and gas line intact. Plumbing is not a big issue if I need to change them a bit.