[Russ Noble]

The fact that it starts easily with a bump start indicates to me that either the starter is not turning it fast enough or, despite all your tests, insufficient spark is getting to the plugs when the starter is operated. Since you say it's a mission to fit a gear reduction starter, I would tend to temporarily wire the ignition only, to a completely separate battery to eliminate the voltage drain scenario.

Also, what happens when you throw a bucket of cold water over the starter motor? Play a hose on it for a minute or so might be a better bet! Since temperature is an issue. You have done that, right?