need some help with a 02 Yukon

[BarryB]

It belongs to a friend of mine. He brought it to me hoping I can fix it, but I basically I am old time stuff mechanic. When I first got it, they said they took it by advance for a free code scan they said it could be a bad o2 sensor. They drive it for a little while because it was just the cel on. wasn't running to bad,the story I got it started loosing power so he was forcing it had up 4k and it wasn't moving. I get it at my place on the back of a rollback. it's running really rich, missing and just running like crap. I pulled three of the four o2 sensors out. pass side looked fine. drivers side the one behind cat was wet with gas. well I put a code reader nothing fancy on it. it first came with p0101 p0102 po300 po420.
heres what I did so far, since it was missing I changed the plugs, and wires, and also replaced the MAF sensor. Did all that now the only codes are showing is the po300. and its still running like crap. The way it sounds almost like it has jumped time, but only one side of the motor and all 4 plugs were black. the other side looked normal. So if it had jumped time wouldn't it effect both sides? since there isn't a dist. on this thing how would you check timing? where is the crank sensor cam sensor? would these things going bad not trip a code reading? I'm stumped for now.