I shoot a weekly military match with a chinese SKS and have a yugo SKS as a back-up rifle. It keep right up with the AR's and M1A's there.
What I did to these rifles.
1.) Checked the headspace to make sure the chamber was tight enough. Look at the throat closely. Make sure you got a rifle worth doing. Same with the crown.
2.) Trim the crown. This is the last bit of rifling before the bullet leaves the chamber. One nicc or some wear and the bullet does not spin uniformly.
3.) Trigger. Remove the trigger group. Wash it with kerosene and a brush. Blow it clean with compressed air. Let it soak in fresh kerosene overnight. Blow it out with compressed air. Maybe brush clean one more time. Then oil the parts. Mare virtually every SKS trigger a ton better.
4.) I do not advise bedding the stock. For one thing bedding a stock works good with rifles that are bolted to the barrel. The SKS is not one of those rifles. What I did do is to make sure the barrel was contacting the barrel evenly. in the front and rear. Keep in mind, the rifle will heat up and the barrel ois held in by multiple points. The best advice is to leave it alone. It doesn't make a big difference.
5.) Sights. Tech sights are a good option. However, if you anal like me and like a clean gun inside and out after every use, the tech sights can be a pain. I prefer the lyman 66SKS sight. They are discontinued as of last year, so they won't be impossible to find. Keep the sights dry, oil makes them glare more. Glare make you groups wider, and it makes them drift towards the sun because of the false sight picture you get from the glare.
6.) If you are a larger framed person, then get a longer stock. A proper stock makes a difference in how you hold the gun. Hold it properly and it's very steady. Don't hold it properly, and your groups suffer. I think ram line still makes the monte carlo styled hunting stock.
7.) Ammo. When I compete, I use H4831 powder, CCI #34 primers, and a 123gr hornady fmj bullet, all loaded to SAAMI dimentions. SKS rifles general have a long throat, so to take advantage of close throat contact, the bullet ends up being too far out. I do have a tight throated yugo sks, and it does roughly 1/2" better with a longer bullet seating. I crimp my ammo, and this helps more than the longer bullet seating. When I am lazy, I use golden tiger. I am a big proponent of using quality hunting grade ammo. Golden tiger is as FMJ as FMJ gets. If using the gun for other than target shooting, get wolf military classic in fmj. It has an empty cavity tip, which acts like a hollow point of soft point. This ammo is much more effective terminally than their 154gr soft points, which are not soft and really do not open up. They'd probably open up with .30-06 velocities, but the SKS or AK's little 30 grain capacity cartridge can not do that. I use sellier and bellot soft points for new brass cased ammo. It is about $10-11 for a box of 20 and is reloadable. I have had zero defects when reloading 3 times. I then put the brass away for safekeeping and then use new ammo. When reloading for other than target shooting, I use hornady 123gr V-max heads.
8.) Clean the gun. Used SKS rifles are so dirty, it is not funny. I push a wet patch of hoppe's #9 followed by 5 bronze brush passes, follwed by a dry patch. It take about 10 rounds of these to get the powder fouling out. Then It takes usually 5-7 copper treatments to get all the crude out. 2 wet patches of ammonia based copper solvent. Wait 5 minutes. Then I put a few drops on a plastic britsle bore brush and push from the muzzle end out 2 times. Then dry patch. I repeat until there is absolutely no blue out of the barrel. When done, I push a water wetted patch twice down the barrel. Then spray some WD40 done the barrel followed by a WD40 wetted patch.
On real dirty new to me rifles, I usually fill a soda bottle with kerosene and put in on the barrel muzzle end. I push a bronze brush down the bore. When it exits the barrel, the brush is dipped in the kerosene. It is a much faster way to get the powder fouling out.
I have recieved a few rifles that were "shot out". I cleaned the guns as described above, then they shot excellent afterwards. One rifle did need a crown cutting due to poor cleaning technique. I bought a few rifles for cheap from people I knew, I then returned the rifle and got refunded the money.
I got an SKS that shoots 2" groups with irons at 100 yards when clean. After about 50 shots, the groups start to widne out. I wait 100-150 rounds, that gun will start to shoot 4-5" groups. That gun shot 6-7" groups before I worked on it with the above steps. Proper barrel cleaning alone got it down to 3-4" groups. Ammo got it down to 2-3". To get to a solid 2" or on a good day closer to 1.5" thats when the rest of the work came in.
Also, the chamber needs to be cleaned. Push a 45 cal brush into the chamber a few times. The laquer coated ammo leaves a coating on the chamber. If you run the gun hot, do not let a round sit in the chamber. The laquer cooks in bad when it's hot.