HUSQVARNA CYLINDER & PISTONS 575XP where can i buy them

Check your muffler spark arrestor.

Sounds like you're timing is off this unit have a timing belt or timing chain it might've jumped to the valves may be opening two early or too late hit in the Pistons

Under the lower intake manifold.

You need to remove the engine from the tractor. un bolt and lift the cylinder head off, remove the ring off bolts on the crankcase closure plate and remove, undo the big end bolts and push the con rod and piston out through the top of the cylinder

Your muffler is clogged,look on line or call your local dealer for part.Here's a link to the parts.

The workshop manual is not available yet, is there anything specifice you need to know, i am quite up on this model and the the replacment 576xp

just looked at a couple of pistons not sure what the letters mean ,,,but when installing the piston make sure the arrow always points to the exhaust port of the cylinder

The H jet needle controls full speed operation and L controls idle operation. Turn both screws CW to stops (lightly), then back out each CCW 1-1/2 turns. This should allow the engine to start and run. Make sure the fuel filter, air cleaner, and muffler are not plugged. Check the fuel lines for decay or damage. Make sure the carburetor fasteners are tight including the hardware that holds the carburetor to the cylinder. Start the engine and allow to warm up, then pull the throttle full on. Turn H needle CW until the engine starts to speed up--proper setting is when the engine 4-strokes unloaded, but immediately 2-strokes when cutting. Allow the engine to idle and adjust L CW so that the engine runs fairly well, but allows the engine to 'follow the throttle'. Adjust idle speed screw so that the chain stops turning, but the engine continues to run. Hope this helps!

Unless really trashed you can simply replace the piston, and re-bore/resleeve the cylinder, that will cost much less.An competent motor bike, or lawnmower repair shop should be able to do that for you, that way you could get a "Kit" online, and simply do that.
http://www.instructables.com/id/How-to-Repair-a-Lawn-Mower-Engine./
http://www.repairfaq.org/samnew/lmfaq.htm
http://www.bbssystem.com/manuals/Small-Engine-Repair-Ebook.pdf

My Husky 359 workshop manual says on Appendix A (USA) says "H" 3/4 turn CCW from lightly seated. "L" 1.5 turns CCW from lightly seated.
Page 36 (every other nation on earth) 1 turn CCW from lightly seated for both the "H & L" needles.
There is no recommended initial "T" setting. (from manual) "The idling speed is correctly set when the engine runs smoothly in all positions and there is a clear margin to the speed at which the chain starts to move". I wager your manual would be similar.
These are initial settings and the saw needs to be adjusted for performance. Start with a clean air filter and a fresh fuel mix @ 50:1 per Husky (1 gallon of premium fuel to 2.6 ounces of quality 2-cycle oil).

This explains performance carburetor adjustment better than I can:
Carburetor Adjustment by Madsens.com
http://www.madsens1.com/saw%20carb%20tune.htm
CW is leaner; to lean will destroy the saw. If over tightened closed (CW) the adjustment screw faces are easily marred; marred adjustments are difficult to impossible to set properly.
Clean the air filter; adjusting with a dirty filter can cause a run lean condition once cleaned and the saw run.
There are 2 wav (sound - idle & full – may not be highlighted) files that I find most helpful, I think you will too.
I hope you found this post helpful.
Lou