[Darklen]

The orange tip can just be painted over. If it's metal, you could remove it (usually held on with a 1.5mm hex key, remove key and unscrew flashhider off the opposite way you usually do it) and use paint thinner, acetone or even brake fluid to remove the paint. Do not use acetone or brake fluid if it's plastic. Alternatively, you can replace it with a black one.

To remove the trigger guard, you have to drift out the pin in the rear and twist the guard slightly to remove. Reverse to install the other one.

To move the battery to the rear, the mech box will have to be rewired as it's front wired currently and you'll have to get a rear battery stock, like some type of Crane or go with a full M16 stock.
The body, buffer tube and buffer tube retaining washer (inside) will probably have to be modified to allow the wires to pass without damage.