What are the best mods for trail riding with a sport quad?

Answer 1

Skid plate - It covers the entire bottom of the frame to protect the engine from getting smashed by rocks, boulders, tree stumps and other garbage in the woods.
A-Arm Guards - Same thing
CV Boot covers & brush guards - Same thing
Body Armor - Same thing
Handguards - To prevent you fingers from getting busted by tree branches.
http://www.powersportrider.com/CGI-BIN/ZCATJPG?catpub=ST0206C
Pick ATV Catalog
Go to - Index
Skid Plate
A-Arm Guard
CV Boot Covers
Chassis Armor
Handguards

Answer 2

I would look into camshaft, intake and carb, and possibly a big bore kit if available.

I would avoid anything that would make it louder. Here's why. The excess noise will annoy private property owners. They can and will stop you from riding on their property. You may be denied easy access to public riding areas if you need to pass through their private property to get there.

The excess noise may also cause people with stock ATV's to also be restricted by your actions.

The best mod would be to purchase an ATV with sufficient power in the first place.