Welcome to the 405th!
These assets are made for foam and/or pepakura, so aren't suited for 3D printing. However, you can find free 3D print files for the helmet here:
Here is the Halo Reach Operator helmet both with and without the B5D0 Optics.Helmet is sized for a 24" head
www.thingiverse.com
I'm not sure what size the helmets are for, since these assets are from the game, but there's a few ways to scale it to your head. You could 3D scan your head (there are free apps/software that do this, like Polycam or Meshroom) and insert the scan and the helmet into 3D software like Blender (also free), then scale the helmet accordingly. Alternatively, the Armorsmith program allows you to input your body dimensions on a virtual avatar, and scale armour based off of that. To test the size, you can print just a thin ring that's a cross section of the helmet and see if that fits around your head (remember to leave room for padding and electronics!)
As for modifying it, the difficulty depends on your 3D modelling skill level and what exactly you need to do.
For a voice filter/amplifier, you may not need to do anything. For a flashlight, you'll likely need to subtract a hole in the model (in 3D modelling this is known as a "Boolean" operation), and may need to also create a hole for wires to run through, if you use any.