Hi,
If you are used to boiling water in a tin, and can no longer use a fire, have you considered replacing the fire with a (say) Coleman dual fuel stove.
It seems to heat a large billy of water faster than I can heat it with gas, and has the advantage that you know how much fuel you have / use.
You can go high tech and use a battery powered pump from the billy if you want.
The stove/billy has multiple other uses, whereas a dedicated gas water heater is some what limited in its capability to multi-task.
cheers