I would be very surprised if the Thielert 1.7 could be run on 12 volts. I would not expect that data would be provided to enable this, there's no upside for the engine data holder to take a risk by supporting such changes. ECU's can be "precise" in design and operation, so I suggest caution. When an ECU is unhappy, the engine generally stops running with no warning. The diagnostics for that engine requires special software, I've done some maintenance check flying with the diagnostics computer plugged in to find a fault.
For that engine, be certain that you understand the gearbox and propeller limitations before you run it. Only a propeller approved for the engine would be safe to operate on it.