A beautiful vessel and heartening to see being brought back into active service.
I'd certainly go for gas-firing, so look through what is already commercially available for just such model boilers. These are outside my personal experience but I do know the field is well covered.
Try for a start, Clevedon Steam, who appear to have taken on the Cheddar Steam name, stock and goodwill. Other, similar firms may be available!
I would not try to build my own gas tank, not for running the boat in recognised model-engineering / model boat events. You can, but the test requirements are remarkably stringent. Have a look at the MELG Boiler Test Code 2018 Vol.3 – and try not to be too frightened. Vol.2 deals with testing the boiler itself, in such scales (<3Bar.litre rates, as I imagine your boat's will comfortably be).
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Paul _ I do type with 2 fingers, but one operates the back-space key to correct the mistakes the other makes!