X, i think the stock jet is a #82 , i think they do make a #85 for it.
to change the jet is simple, remove the bottom bowl of the carb (3 screws), unscrew the jet (one is a sprayer, the other a jet) ..replace, re-install. nothing major at all.
*another restriction possibility is*
on the top of the carb the little black cover thing, some have metal covers but anyhow..there may or may not be a small screw in the top of the cap on the under-side, make sure this is gone already (it probably is) but there is a spring on the underside of that cap make sure you dont lose it. its not a tiny one so you should be ok.
if there isnt a small screw in the underside of the plastic carb cap then that restriction is already been removed or came from manufacture without the restriction. (some do some dont)
also..Try running with 3 rollers at a time see if that improves your take off, i have three 16x13 x 6gram in the one i usually ride the most, (4 stroke gy6 50cc)after 4000 miles no problems,but when you change the rollers next time use the other 3 slots in the variator. I have MRP rollers but i have the stock sliders. try it with 6 then try it with 3 and see if it improves the takeoff.