Build Your Own 12 VDC Engine/Generator

How to build your own 12 VDC engine/generator from a “lawnmower engine (or any other small horizontal shaft engine)” and an automotive alternator for use in concert with a renewable energy source (solar panels, wind turbine, etc.).  The idea is to use this homemade genset to help keep your off-grid battery bank charged when the renewable energy source(s) don’t quite get the job done alone.