I recently bought a very low hour 375, I changed the plug although the old one was good, put on a new K&N air filter. It was missing bad at top speed so I replaced the above stuff and it did not fix the issue. I think it may be a main jet problem, but I am open to any suggestions. This is my 1st artic cat, but one of many toys.  Any and all advice would be greatly appreciated.
Ranger  

before you go changing jets try to adjust the fuel air mixture screw a little.

by missing do you mean it is cutting out?

if so then it is most likely not getting enough fuel in the mixture.
turning the fuel/air mixture screw to the left will increase the amount of fuel in the mix.

be sure to mark where the screw is setting from originally, and keep up with how far away from that you go.

turn it in 1/4 turn increments let it run for a minute or two then see if it needs more.