E&L Essential AK74 - need upgrade tips

Hi!

I've recently got my hands on a used E&L AK74 Essential, which shot pretty well i guess, however after replacing the old hopup rubber to a brand new maple leaf, the fps dropped significantly (It makes about 0,4J now). I might have screwed something up with the air tightness of the whole thing, since i tweaked with the gearbox a little.

Im new in the community, what parts should i buy and what upgrades should i make to make it work just fine?

https://preview.redd.it/52dx0zkurj6g1.jpg?width=957&format=pjpg&auto=webp&s=5aff9719d56fa38ac7eaa06137f69c66c69ef1d2