I think the check list for WR125 owners is pretty much what Kelly indicated:
1) When you purchase the bike just add ~$250 to the price and order the PWK airstriker at the same time. The JD kitted PWK from Motosportz is the best way.
2) Adjust your linkage in the picture to the top.
a) IF after you have it running like a raped ape you want to change
engagement rpms or the progressive nature of how the pv's
open then invest in the spring kit.
3) Add the protection of your choice and hopefully an aluminum skid plate made to fit.
4) Add a larger tank if Husky doesn't expand the current 2.3 gallon tank.
5) After break in at the very least change your fork oil and bleed the inner chamber. I wanted more and sent mine out for some work, several very talented people to choose from- LTR, ACE, WER, etc.
6) For the auto clutch crowd, EFM does a bang up job for the Husky 125. I highly recommend them.
7) For pipes, while the stock unit works ok, I feel the FMF Fatty combined with the TC2 silencer adds a little everywhere. Kelly's Doma works phenomenal as I have heard that from others with this pipe on other makes 125's( of course you have to find one).
8) The new 125's have incredible brakes, spit you off into the tree in front of you type brakes. Add these to some Motoz tires and you can learn all about the oh s$%t moment. Controlling the rear brake is made much easier with a LHRB that works in concert with the pedal. Rekluse makes a kit that works well if expensive. EFM also makes a kit that is much less expensive but you will need to combine this with a two finger clutch lever sold by Rekluse.
9) All the bike prep tips for other models apply: grease everything that takes grease before you ride. Tighten all the usual suspects like sprocket bolts, motor mounts, etc.. Water proof your air box by either taping off the bottom water drain ports and slitting your tape or plug them with compressed airfilter foam, stops water and dirt from just flowing into the airbox.
10) My opinion is that the 144 adds that bit extra everywhere that makes a great bike close to perfect. It hauls my 230LB riding weight up pretty severe climbs any way I want to. Putzing on the bottom end, mildly motoring up quickly in the mid-range or absolutely hair on fire freaking out of control on the pipe grabbing gears as fast as you can. You can spend a little more and keep your original top end for resale and order the Husky oem kit, spend $300 less and get the Eric Gore kit, or do it yourself by ordering an 05-09 YZ144 Vertex forged piston kit from Oemcycle.com and send it with your power valves and cylinder to Millenium with a total cost of about $320. Sending your stuff to George Erl at Uptite Husqvarna is also a fairly painless way to accomplish this.
11) Maybe Coffee in his spare time

can attach links to the pertinent threads that deal with all these things.
12) This really needs to be a living thread that can be added to as more "Stuff" is developed, learned, or comes available.
Walt