I would shift, scale, rotate, and move both images to develop a unified look. This just requires some compromise from the original "perfect" right-side design. There is also no rule that both left and right images have to be in the exact same spot left-to-right, particularly since this is a custom one-off.
You can do whatever you want with it. It does not require symmetric perfection.
Consider this: The production bikes lack these types of cool designs specifically because the makers are unwilling to make any compromise to the symmetry. This is an instance to perhaps throw out that tyrannical rule and let your artistry flow.
Right you would never reverse it, the reason being, as a symbol it would be meaningless.
Look how much this side was compromised, the text is cutoff from view, so that the left side could be as large and as perfect as possible. And a lot of people see this one in public.
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