Gear box issues

Hi guys long story short Iv fitted a 1.3 gear box to my road rally feli and its rather noisy, sounds as if its input shaft bearings so has anybody here stripped and rebuilt their gear box with new bearings and gaskets? If so is it a simple straight forward job (gear boxes aren't my fav thing) are new gaskets available and does anybody know what bearings I need to get as Id like if poss to have everything before I start as time is of the essence, or on the other hand I have the smashed 1.6 gear box I swapped out are they the same bearings and everything inside (so I could strip and size bearings out of that box for the other one?)

Cheers guys.