I'm new to mnt biking and I'm reading all I can trying to learn. My problem right now is my drivetrain. The front chainrings and the rear cassette are in bad shape. Some of the teeth are half gone and the chain slips sometimes.

Since it needs replaced, I was considering changing it up. Right now it's and 8-speed with 3 chain rings. I think I would like a 9-speed with two chainrings and a bash guard. I rarely use the large chainring and I regularly hit it on logs.

I have a Trek 4300. Is it possible to use the stock crank or would I have to buy another crankset? I'm laid off so I'm looking for the cheapest fix. Thanks!
https://www.trekbikes.com/us/en/bikes/2006/archive/4300

Going from 8-speed to 9-speed would require a new shifter, cassette and a chain.
Shape of teeth is a difficult way of determining drivetrain wear, as teeth are shaped differently deliberately to make shifting smoother. But chain slip is a decent indication of drivetrain wear.
As long as rings aren't riveted in place you should be OK with simply replacing the biggest ring with a bash guard of the same BCD and number of arms.
Or cheaper yet, use the limit screws to lock the FD to the two smaller rings and keep using the biggest ring as a bash guard. Won't cost you a thing, and it will work just fine.

Thank you! Good info!

Unfortunately, mine are riveted together. I thought about drilling out the rivets and replacing with bolts, but I wasn't sure if replacement chainrings would have the same bolt pattern. My current chain rings are worn out, so I'm going to have to do something. Do you think I can replace the rings on mine?

I like your idea of using the biggest ring as a bash guard. I have the tools to remove the teeth from it and make it smaller and so that it glides. Right now, the teeth stick in logs and there's no gliding to speak of. I wasn't aware that I could adjust the FD to only the smaller rings.

I was aware that I'll need new shifters if I go with a 9-speed. I figure since I would be losing a chainring, that it would be good to get an extra cog and make up some of the loss.

If I don't have to replace the crankset, then I'll just get another 8-speed for now since money's tight. I'd much rather buy two CR's and a cassette.

If your chainring teeth are actually messed up, and it's a riveted crank, you'll need to replace the crankset. And perhaps the bottom bracket. You could go with 2+bash-guard and just keep the 8-speed setup.

Does the chain slip on the front or rear gears?

Check ebay seller wheelandsprocket. (may be wheelsandsprocket) they have a bunch of NOS cranks for cheap, many include the bottom bracket too. I'm talking 25 bucks cheap. May just be new triple cranks and may not solve your problem with hitting stuff but certainly as a way to at least get the drive train running properly its cheap.

Thank you guys!

I'm pretty sure it's the rear, but I've not actually seen it do it. It usually does it on the 5th-8th cogs. The CR's have teeth missing though, so they need to be replaced.

Would these parts work on my bike? This is my wish list.
https://aebike.com/cart.cfm

cant tell you, that link brought me to my wish list with them! give us the links to the parts.

I took a closer look at the ebay store i recommended to you, I think word got around, they have very little stuff left and all of the MTB cranks are gone but there are definately ones to be had on ebay

Sorry about that.
https://aebike.com/product/shimano-de...w8421-qc30.htm

https://aebike.com/product/shimano-hg...h4030-qc30.htm

https://aebike.com/product/truvativ-s...r2583-qc30.htm

Yea, I looked at his stuff on ebay and found a lot of road bike stuff. Oh well, thanks for trying!