Adding Reverse to ’04 TRX400EX – Possible?

This may be tougher to accomplish compared to my previous question.

How possible would it be to add reverse to a 2004 400EX? The newer models have the gear, and from what I’ve seen mechanically they look similar.

What would be required in order to do this?

2004 400EX (no reverse)
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2005 400EX (with reverse)
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