Assuming you've cleaned the barrel and adjusted the hop to give the best range, and checked the hop unit is seated correctly, if its still not great I would suggest you concentrate on sorting the inner barrel and hop rubber first before looking at the gearbox which can be a minefield if you've not opened one before.
Everyone and his dog will have their own preferred brands etc but I find a ZCI or Core 6.02mm inner barrel (£15-20 from AK2M4 on this forum) and a standard £5 TM hop rubber (from Firesupport) a very cost effective upgrade personally. I used to use the dearer Madbull Black Python barrels but these are £10 or so dearer and don't offer anything over the ZCI/Core ones tbh. The Hop rubber on its own may give you better results, there are many more expensive ones out there and having tried a few always come back to the TM one, but as said these are just what I use based on experience and others will have other ideas.