My advice would be to get a cheap clone like a JG. Because of the low cost, there is low risk in case you screw something up, they are completely on-spec so you can replace parts easily, unlike your KWA, and they have pretty good parts stock to begin with. The internet is your friend. Airsoftmechanics.com is a great place, but rather advanced. Sites like airsoftretreat would also be a great place to go. Youtube has many disassembly and reassembly videos, but many of them beyond basic taking things apart and putting stuff together don't know what they're talking about from what I've seen, so take it with a grain of salt. They are good at showing how to put the gearbox back together though. Vindi's DMR guide has many assembly and tuning tips, even though it is mostly DMR oriented, so that's good to check out too.