Detuning

What can be done to 'detune' a YZ to make it more trail friendly?

What year and model YZ?

2002 YZ 125

My opinion:

I would first try the bike for a couple of rides and then decide, as well as search the threads on the YZ forums (like ThumperTalk, etc.).

Areas today's riders will modify are:
Lower gearing (smaller front or larger rear sprockets).
FMF exhaust pipes that add a bit more low end torque.
V-Force reed blocks.
Flywheel weights.
Big bore kits.
Having the suspension modified for trail use vs. motocross (which typically makes the bike feel stiff and unfriendly on trails unless you are a fast rider who likes that feel).

The main advantage with a YZ125 on the trails is the light weight of the bike, especially with how light it feels once you are in motion.
This is due to the relatively small physical size of the engine and the physically small size of the rapidly spinning parts (crankshaft and generator flywheel).

The main drawback of a YZ125 on the trails is that it doesn't have a lot of pulling power and torque if not in the powerband - you need to know how to ride the bike.
Riding the bike well is priority No. 1.
Typically, the smaller the displacement of the dirt bike, the more you need to rely on your forward momentum to conquer obstacles like gnarly hills and such (AKA not coming upon a tough-looking obstacle, panicking, and slowing too much).

Do search on the yamaha 2 stroke section on thumpertalk.
This has been covered a lot on there.

Yz 125's dont have much lowend, the useaul detuning mods for the larger two strokes dont do much as it just wont pull down low.
Pipes ands reeds dont do much at all as the stock stuff is really good.
Flywheel weights do help tho.

I'd just throw an auto clutch in it and ride it.