increasing player's speed when entering booster and a speed upgrade system - unity3d

Question

I'm building my first game in Unity using my limited knowledge, youtube tutorials and troubleshooting on google and even though i have a couple of idea's on how i could do this i can't seem to get it to work.
For one of the larger levels i need speedups that boost the players speed when driving over them. This is seen in a lot of racing games but to clarify i found this video where this type of boosters is used https://www.youtube.com/watch?v=GQ1FLPBw1FE.
I've tried making a script that would remove my current movementscript and replace it with a faster one, i've also tried making a script that would detect collision with the player(tag) and add force to it. Both didn't work i've also been thinking about using timescale but i don't want the rest of the world or scorecounters etc to speed up, just the player.
this is the basic script i am using for playermovement.
#pragma strict
function Start () {
}
var forwardSpeed:float = 35;
var turnSpeed:float = 1;
function FixedUpdate () {
var forwardMoveAmount=Input.GetAxis("Vertical")*forwardSpeed;
var turnAmount=Input.GetAxis("Horizontal")*turnSpeed;
transform.Rotate(0,turnAmount,0);
rigidbody.AddRelativeForce(0,0,-forwardMoveAmount);
}
and since the player's vehicle is customizable i also want to make an upgrade that permanently increases the players base movement speed by a certain amount. i would do this by putting in an if(upgrade is active)
increase its speed by x;
this would have to stack with the booster
if you guys have any idea's on how i could do either or both of these things please share and thanks a lot in advance!

Answer 1

Instead of replacing the movement script with a faster one, add in an extra variable to the players movement.
var forwardMoveAmount=Input.GetAxis("Vertical")*forwardSpeed * boostSpeed;
Where boostSpeed regularly equals 1, but when the player collides with a speed boost push the value up to something higher like 2, then after a short while it gets reset to 1. As for the player speed upgrade, follow the same logic, except permanently, so it would look something like this:
var forwardMoveAmount=Input.GetAxis("Vertical")*forwardSpeed * boostSpeed * upgradeSpeed;