First, play with the RIF and see what aspect makes you feel insufficient on the playing field. Do you feel like you need more range? Better accuracy? Faster rate of fire? Snappier trigger? Once you know what you want, here's what I'd suggest doing...
The first two performance boosts I'd suggest are a good quality LiPo battery (11.1v or 7.4v, with 11.1v being much faster), for gains in ROF and trigger response if that's what you want. Heavier, higher quality BBs will net you gains in range and accuracy, as well as being able to cut through wind and foliage better.
As the others have set, motor and hop rubber are your first and most effective parts upgrades. These two upgrades cover range/accuracy, and trigger response/rate of fire. You can pair them with a LiPo battery (11.1v or 7.4v, with the faster being 11.1v), and heavier, high quality BBs for greater gains in both departments.
I would personally suggest doing just the hop rubber first, as you'll be able to observe the actual performance change. G&G hop units are already really good, and they fit maple leaf hop rubbers great. I would suggest using a maple leaf rubber in between 50 and 60 degrees (look for Macaron or Super Macaron), and replace the original soft nub with a harder piece of plastic or even a piece of 6mm tubing laid flat in the hop window.
You can polish the stock barrel with some metal polish and a drill (use a rag on the tip of a cleaning rod), and get a decent performance boost. I'd suggest doing this before buying any upgrade barrel. Upgrade barrels are really a minor upgrade--a decent polished and clean stock barrel (as long as it's not bent or messed up on the inside) will give you 80+% of the performance of any upgrade barrel, and sometimes more. For example, the stock barrels from Krytac and Arcturus are just as good or better than any upgrade barrel I've used (Prometheus, Lambda, etc). Not all stock barrels are created equal, but there is no advantage of having steel over brass in most circumstances.
I would also suggest wrapping the barrel in tape, and likewise stabilizing the hop unit with tape or plastic shims. These are extremely cheap and easy accuracy upgrades that I do to pretty much every build.
For a motor, I would suggest ordering a 25K neodymium motor from ChiHai Shenzhen, which should cost around $30. Either that, or take whatever suggestion the more local members of the forum give you. There's not too many horrible options for a motor, and they know local retailers better than I do. ?
