How to make coaster brake work backwards, or disable?

Coaster brake hubs are complicated. There is usually a threaded component inside the clutch mechanism. To make the coaster brake work when flipped, you'll need to replace it with a new part with the threads reversed.

You'd be better off with an electric motor. PowerWheels drivetrains just use basic brushed motors just like power tools, typically 700-series sizes.

There is a probably very useless potential discussion here about which internal parts of the hub would have to be remade to do this, i.e. if you machined a reverse-threaded driver to take the place of the original one, what is the next thing that would go wrong after that. Barring those kinds of considerations, there is no way of doing this because you're reversing the motion of the driver and thus there's no way of getting it to drive the shell.

Do it with left drive BMX parts.

A coaster brake requires a reaction arm bolted to the frame, in order to work. By removing that arm completely your coaster cannot apply.

However you're still carrying the mass of the shoes and heavier hub shell around. it might be better to find a normal bike wheel, and store the coaster wheel for later reverting.

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There have been "rubber clamps" that use a lot of bolts to secure a cog on the left-side of a hub. They are often quite large, 50-120 tooth count to also provide some gearing reduction. This increased size means the cog can be clamped on to the bike's spokes as well for support. Chainline straightness is always an issue.

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Aside, a 49cc motor might be overpowered for this application - a 20" frame implies about a 10 year old child, and they're not exactly large. A tiny ~20cc engine is all they need for puttputting around the garden - its not like this would be legal on the road.