- Use the correct coupling rods. The ones without the extension for the mechanical drive. That's easy, just make up the correct ones from the etch.
- Turn the hole for the crank drive into an inverted U shape. Blank it off a bit and put the bolts on that would hold the gearbox in real life.
That second one is more "entertaining" on a chassis that has all the mechanical bits fitted when you don't want to take them out. Some careful work with the piercing saw was required, not just to make the U, but to remove the carefully soldered in bearing as well. I was blowing swarf away at every other stroke to keep the stuff out of the motor.