These are great little engines, I've had more than a half dozen. They are precision little machines and a bit more complicated than the average 2 stroke, however they are simple to work on if you know what you are doing.
Nowhere in the factory manuals does it mention octane. It says "Automotive unleaded gasoline". I've spoken with a couple of Robin America engine techs and they recommend 87 octane. I don't think it's the cause of your problem but these engines have a tendency to carbonize in the upper cylinder and piston crown. A higher octane gas will only exacerbate that problem.
i think it was gradual, when i stored it for about a half a year, and then brought it back out, it wasn't the same.
Sounds like you already have elements of your answer in your own question.
How did you store the engine? In an outdoor environment in the winter? Did you service the engine for storage before you put it away?
How did the plug read that you replaced? You can also shine a bright light into the plug hole and read the wash on the piston at TDC.
i know how to get to the valves but i don't really know how i should adjust them?
i tinkered with them last night, and i got it to go to full power under no load without dieing sometimes, but it still doesn't have that nice purr when it idles like it used to.
I wouldn't do anything. Not until I felt comfortable with what I was doing.
You said you didn't know how to adjust the valves then you tinkered with them. Not good. Valve adjustment is part of regular maintenance and not very difficult but not something to tinker with.
My early guess would be, and it's just a guess not knowing more about the particulars, is your problem is simpler than you think, fuel gone bad or the fuel system contaminated or compromised in some fashion.
Here is a pdf of the service manual. It contains the valve adjustment procedure and tolerance. You simply have to prep yourself to get it right and the prep is harder than the actual adjustment if you've never done it.
Do you know how to find TDC?
http://www.robinamerica.com/media/manuals/128450729394630000.pdf