This would depend on what type of oil you will be using for your premix. The oil base stock is either petroleum, semi-synthetic or synthetic oil and is mixed with gas at a ratio ranging from 16:1 (petroleum) to as high as 100:1 (synthetic). My 2 stroke I have is a 2001 Kawasaki KDX200 that I run Yamalube 2R at 36:1 with no problems. Although synthetics say you can mix them at 100:1 I would not, that seems way too lean to me.
I personally use 32:1 of amsoil and regular unleaded gas for my ktm 125 sx 2003.
.7 L
My manuel recommends 40 to 1on my 2001 ktm sx I've never had any problems with it..
There is no fuel/oil ratio, the 03 KTM 450 EXC is a four stroke. See link below.
You can find that info , as well as a lot more, here: http://motocross.transworld.net/2003/07/09/2004-ktm-125-sx-200-sx-250-sx-and-525-sx/
.7 L
32:1 ratio
KTM recommends Motorex Top Speed 4T15W50 for the 125SX.
my 03 manual says for the ktm sx 125 20w-40 .7 liters.
.7L
50/1
.7 L