snowmobile lost power and has a knock?

[gyro]

i have a 2001 arctic cat thundercat 1000cc. my brother got it stuck in a snowbank and gave it hell trying to get out. i have half the power it shakes and makes a knocking sound when its being driven. it doesnt do it at an idle. i changed the plugs and i am still getting spark. belt is good. had some people look at it and no one can figure out what it is.