Leostick is (as the name suggests) based on the Leonardo, but in a usb key form factor, with the ATMega (not a 328 but don't recall which one) being SMD.
High side switching will be required as the fuel injector may connect its low side to its stainless steel casing, and in a metal water heating aparatus which is grounded to the rest of the electronic system, it won't be possible to disable the output.