Can I upgrade my 2007 front suspension? I just bought a 2007 Trek Fuel EX5 used but the front suspension dial is cracked and makes it difficult to change the give. The current forks that are on there are RockShock Tora's. Would it be possible to change this out knowing the tire and tread size?

Edit: Thanks for the input guys. You make a good point Matt, I paid $350 for the bike so it might not be worth it, but the front suspension is at nearly full give so I’m a bit worried I’ll constantly bottom out. 

Knowing that I’m not eligible for the upgrade program, how much do you guys think it would cost to buy the parts used/new and which one of those is the best option?
Short answer: Yes easy.

My recommended process:

If you are the original owner of the bike check out SR Suntour's upgrade program.

https://www.srsuntour.us/pages/upgrade

The RAIDON XC-LO-R 26 with a straight steerer would be a really good fork for that bike at $199 down from $299 with the deal.

If that isn't appealing you'll want anything that has support for a 26" wheel, 100mm of travel, and a straight 1 1/8" steerer. I doubt you'll be able to fit anything over a 2.3-2.4" tire in there.

Note: For the fork from SR Suntour I mentioned, it has a 160mm flat mount support for brakes, so you'll probably have to upgrade your front rotor to a larger size as well.