Upgrading G&G MP5 A4

[Masamune888]

HIya guys,
I'm looking at getting some new internals for my gun since I'm in Hong Kong, but I'm not sure where to start. Its a G&G MP5A4 (with plastic body) and so far I'm thinking that a tightbore barrel is probably on the top of my list. Any advice for what I should do would would be greatly appreciated