I agree with Mark that if there is no company around to indemnify it many contractors won't risk the liability of hacking in a fuel-change on this equipment.
In a dank basement in Saugus, MA a heat pump water heater might actually be a better choice than a Navien. A heat pump water heater does double-duty as a dehumidifier, often need 8-10 months out of a year to keep basements from smelling in that area. It'll drop the average temperature in the basement by a degree or two, but it'll drop the humidity by quite a bit, turning the heat of vaporization of the water in the air into sensible heat in the water in the tank, whereas a dehumdifier turns it into sensible heat in the room air. Operational costs are comparable to a gas-tankless, but lower than a tankless + dehumidifier.
I'm not sure if
MassSave will subsidize converting from a propane water heater heat pump or natural gas, but it's worth investigating.