What happened when it used to cut down in speed? Did the revs just die or did it splutter and run rough?
1st check if you got a spark at the plug, take the plug cover off, pull out the plug, push the plug in the cover and hold the end of the plug onto a nut or bare bit of the frame to earth it, now get someone to crank over the electric start and see if you get a spark jumpng from the tip of the pkug to earth.
Checked the battery box isnt full of water and shorting the cdi? Or the cdi has corroded connectors?
I take it you opened uo the cvt to have a look inside? Thats the simplest thing to check 1st. Open it up and check the belt looks ok and isnt frayed or splitting on its edges. If you got the tools take off the variator and check all that out and the rollers too. Alao check the rear pulley looks ok and again if you got the tools take it off and make sure it moves freely and is all ok. I had a torque drive face snap off the pulley before on mine, just sat there wobbling.
Could also be dirty carb/carb jets, dirty fuel filter, dirty fuel line, manifold leak or a load of other things. Have a look and let us know what you find.
Good luck.