Sailor, mine was originally wired for 3-ph, but I rewired it for standard 1-ph and ran a line out to the garage on a 50 amp breaker. It would heat up faster with 3-ph, but I'm not boiling anything, just heating milk up to temp.
Running a gas line is also a good solution, but I don't have gas on my property, else I would have. If you do that, make sure your installer or you pay attention to the gas pressure and use a large enough pipe, and make sure the incoming line from the company has high enough pressure, or request them to bump it up.
From an electrical perspective, you should be able to rewire most of the kettles, it's just that once you get above a certain wattage input requirement, it's not practical or efficient. I think up to 40 gals is OK on a 240 line at 60 amps max (14.4 KW). If you find a kettle you like, let me know and I can try to figure out if it's possible to rewire it to single phase. Up to 40 gal you should be fine, so long as you can dedicate enough juice to the circuit.
You can also get a phase converter and if the voltage is off, a buck/boost transformer instead of rewiring the kettle.