There are many sites that give hints and tips about how to make your own inertia dyno but all that I found earlier were for motor bikes, go karts and cars. However the principle is the same for all of them so any electronics mentioned would be suitable. One site offered the computer programs, another had the maths behind selecting the correct size flywheel etc. The flywheel is the most critical part because it has to be sized to the expected HP of the engine so that it takes around 10 seconds to spin up to full speed from idle. Full speed means around 30,000 revs, not the absolute maximum speed.