Let's get this right.
Daewoo was an independent manufacturer in S. Korea until they went bankrupt and GM bought them, now they are still manufactured and distributed under a Chevrolet emblem in various countries.
The codes you posted has nothing to do with O2 sensor failure.
The P0300 is a multiple misfire code, which could be the result of the Camshaft Position Sensor failure, so forget about O2 sensors and focus on the Cam Sensor and its circuits now. There is a big chance that the sensor is failed and needs to be replaced, but wiring or computer problem could also cause this.
Here is how the CMP sensor works:
The Camshaft Position (CMP) Sensor is used to correlate crankshaft to camshaft position so that the powertrain control module (PCM)/engine control module (ECM) can determine which cylinder is ready to be fueled by the injector. The CMP is also used to determine which cylinder is misfiring when misfire is present. If the PCM/ECM receives an intermittent signal from the CMP, then the CMP Resync Counter will increment. When the PCM/ECM cannot use the information from the CMP sensor, a Diagnostic Trouble Code (DTC) is set, and the PCM/ECM will fuel the engine using the Alternating Synchronous Double Fire (ASDF) method.
Since you said the timing belt is in a very bad condition it may jumped on the sprocket and this may be the cause of the cam alignment. You can check this by lining up the cam and crank marks .
The good news is that there was a recall on the cam sensors (they tend to melt and cause fire) on the 1998-2002 engines and the dealership should replace them for free. The bad news: good luck to find a GM dealership who will honor the recall and do this repair for you.
You can call the NHTSA and ask them about the problem, they may be able to help you. Here is the recall info - including the NHTSA phone number:
Recall 04V357000: Cam Position Sensor Melting
Make / Models : Model/Build Years:
Daewoo / Lanos 1998-2002
Daewoo / Leganza 1998-2002
Daewoo / Nubira 1998-2002
MANUFACTURER: GM Daewoo Auto & Technology Company
NHTSA CAMPAIGN ID Number : 04V357000 Recall Date : JUL 12, 2004
COMPONENT: ENGINE AND ENGINE COOLING
Potential Number Of Units Affected : 11576
On certain passenger vehicles, the camshaft position sensor may melt resulting in a burning smell and visible smoke, which may subsequently lead to the melting of the camshaft cover and the camshaft position sensor wire harness.
This could possibly cause an underhood fire.
Dealers will replace the camshaft position sensors and install an additional fuse. The recall began on September 7, 2004. Owners should contact Daewoo at 1-877-362-1234, option 6, or find a local dealer through their website at www.daewoous.com.
Customers can also contact The National Highway Traffic Safety Administration's Auto Safety Hotline at 1-888-DASH-2-DOT (1-888-327-4236).
Now back to the timing belt question.
You should replace ALL of the TIMING COMPONENTS. The idle and the tensioner pulley,water pump, cam and crank shaft seals also - beside the timing belt.
They all could cause a potential engine damage in the case of a failure.
In case you need instructions you can find great repair manuals with step-by-step instructions, diagrams, parts locations and other important information here: http://bit.ly/mitchell_repair_manuals_online
Or here: http://bit.ly/AllData_Repair_Manuals_Online
BTW, the replacement interval is 60,000 miles on the timing components.
Isn't RepairPal great?