I don't understand why there's a diode at the input.
Adding a 7812 at the input wouldn't serve any purpose. The 317 can take an input up to 40V. Also, the common 7812 has a 2V dropout, it would only put out 12V if the input was >14V.
Be aware that a good deal of heat can be generated, it's proportional to the output current. Heatsinking would be a good idea. There's also no charge termination, there's no "simple" way to do charge termination.
Actually, best thing might be to bring your battery charger from home and power it with a power inverter.