I absolutely LOVE converting Ertl toys ,they are the best value for a cast metal toy & lend themselves so well to be converted to be ran by steam toys. Just as you have said ....I don't care if they are or aren't supposed to ran by a steam engine , just like a windmill....if its fun that's all that matters.
You'll probably find that the axle bores work pretty well as is. The flywheels and pulleys are usually held in place by knurling on the axle ends , simple twisting while pulling on them will generally remove them.
Have fun with it & I look forward to your video 😊👍