I just bought some Longlife spark plugs from VW. They make a big difference to my G40 but then it did have some random ones in before which def weren't the right ones.
I also changed the leads last weekend, but got those from GSF. Sorry, I don't know the make (I do now - Beru), but I do know the part no (92620H). Think they retail at around £25.
Air filter - standard from VW but I don't have any other engine upgrades. I will probably change when I do.