Ive drained 1liter out of my yz 125 so wot should i put back init?

SOURCE: 1999 yz 125 service or owners manual

www.bikebandit.com and or www.extremepowersports.com

these both have detailed pdf files you can look at.

SOURCE: what oil should i have in yz 125 2 stroke

The clutch and gearbox share the same oil. The oil for the motor is 10w40. Stick with the major brands; Pennzoil, Quaker State, Mobil, Yamalube or Shell.

SOURCE: what spark plug for yz 125?

I have a dennis kirk book, the correct spark plug is a --BR9ES--(NGK)

i always used SAE 10w-30 oil on all my dirtbikes,
no transmission fluid

for the chain you should move the back tire back until the chain is at a snug tension

SOURCE: how do i decode yamaha yz 125 engin id number

You don't decode it. It is only a production run series number. Mororcycles are like cars in that respect. It is the frame number, AKA the "VIN number", is what carries all the information. Call a dealer with the frame number and he can tell you year, paint color, specific model and more. They may have a problem if the bike is too old. Yamaha changed to a newer code system a few years back.
Can I get a “very helpful” rating on this answer?

SOURCE: YZ 125 kick starter doesn't spring back

Spring is busted. If you aren't comfortable with piles of extraneous parts floating around, take it to the shop.