You have a few options here, It may not be as bad as you think.
You can replace the unit with a a Mobile home approved (sealed) water heater. They do get a little expensive, $600 is a ball park price here for the unit Plus installation.
You can go with an electric unit. Initial cost will be much less, but you will most likely need to add an electrical circuit to handle it.
The third option would be a demand type unit, probably the most expensive option.
"If I can't fix it, I can screw it up so bad no one else can either."