What would work:
Saddle, pedals, drivetrain (assuming rear wheel for the DH bike has the same type of driver), handlebar, grips, brakes, rotors (provided they’re both 200mm at least).
What won’t: fork (dh bikes usually like a 200mm dual crown), wheels (spacing for the fork and rear are different), cranks (bb shell is 83mm on dh bikes instead of 73mm), rear shock.
Possibly: seatpost (get a straight post for the dh bike, and stem (direct mount is better). Tires usually are 2.5 on a dh bike, not 2.6.
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