YZ125 essentials?

What year YZ125?

2005-2006, make sure you do the 2007+ shift stopper replacement.

2006+ are really solid stock. Put in springs for your weight, maybe a little jetting fine tuning and let it rip.

Larger footpegs from the newer models if you have an older model.

After that, my mods were larger front rotor (newer versions have 270 stock already), skid plate, rad braces, Motion Pro Titan throttle tube.

SKF fork seals and wipers
Enzo compression adjusters
HD 520 chain
Vforce reeds
Raw pipe

2006 and up are very solid bikes stock, but they need a little bit of attention here and there.
VForce Reeds are a must. V3 or V4, doesn't matter too much on which one.
Pipe and Silencer. Depends on personal preference, but I love my set up of the PC RV Works Pipe with the 304 Silencer. Has a lot of low to mid grunt to help with cornering, but winds out to the top very quick. You just have to be very active with your left foot lol.
Personally, I don't run it, but a VHM cylinder head with the MX insert will lower the compression while making the powerband more useful. Plus you can still run pump gas.
49 tooth rear to keep yourself in the powerband, but try out a 48 rear too.
Fine tuning your carb with a JD Jetting kit or any jets in general.
2015+ foot pegs if the YZ is from 2006-2014.
The power valve has a tendency to come loose so checking up on that helps.
The fork seals love to go out so replacing those with a suspension service will save you a massive headache.
Pay attention to your steering bearings. The seal itself does a horrible job keeping water out so the bearings will wear out quick.

VForce 4 reed block and a HGS pipe and muffler will make it scream. Other than that, spring for your weight and rip it

i had a '06 so i replaced the shift stopper and i used a Boyesen rad valve, Get cdi, Modified HGS exhaust by Kemea EMX team and a VHM head with different inserts.

I've been rocking the SKF fork seals on KYBs and SHOWAs. GREAT upgrade in feel and longevity. Somebody here turned me onto them I think - great lead.