I have a L120 JohnDeere tractor and it is losing power after I use it a while. I put a new drive belt on it but didn't help. I'm thinking of dropping the transmission and changing the oil, but was wondering how hard it would be to rebuilt the tranny? Does the manufacture of the transmission make a rebuilt kit for it, and how hard is it to rebuild one?

How do you know it's the transmission? Download a manual and trouble shoot other maintenance issues first before you tackle a transmission. You have carburetors, plugs, wires, clutches, bad gas, and a load of other things to check first and a manual to guide you through the process will even help you with a transmission if indeed that's the problem.

John Deere L120 Lawn Tractor - Free Product Manual (http://www.freeproductmanual.com/john-deere-l120-lawn-tractor/)

Services and Support from John Deere (http://www.deere.com/wps/dcom/en_US/services_and_support/services_support.page)

Hi,
I do not have the links for the YouTube videos, but there is a two part video of replacing the trans pump and motor on the unit. Deere sells the parts as does TuffTorq (Kanzaki). Depending on the age of the unit, it could well be that the new parts will have the updated pistons.
You may want to check out the videos by doing a search. You also probably want to check out the unit itself by butting it up against something and seeing if it will spin the wheels or what seems to be slipping. The idlers do have a bit of adjustment as they are in slotted holes.
Peace,
Clarke