ITR Mainshaft? need a new one questions and How to tell usdm from jdm?

Mine is bad one of the teeth are gone. Can i use a gsr trans and just switch the lsd and be done or can i use the gsr mainshaft?

Do i need just the ls 5th gear and syncro to install it so i have a higher 5th?

I know its a lsd trans thats easy but how do i tell if its jdm or usdm?

It had j4d lsd stamped on it and it says s80 and some number after that on the case... How do i tell if the final drive? On both the case and lsd/internals? As before i bought it it was pulled apart?

pretty sure j4d is jdm only.
Just wanna make sure which fd it is and what parts i can use to fix it

Thanks for your help.