1996 Suzuki Sidekick No power when timing set correctly

I bought my son a throw away car for his first year of driving to learn about car maintenance and driving skills. So the car I picked out had just enough things wrong with it to give him an idea of all the things that can go wrong with a car in a normal life. No point in having him learn each thing over 10 years. All at once makes more sense to me. This car will live in family legends for generations.

The last remaining performance issue is the lack of engine power when the timing is set correctly per the specification of 5 deg TDC. When set correctly, the engine will idle and accelerate when in park but when under load, the car goes nowhere. Pushing the accelerator all the way to the floor does nothing, no change in engine speed. If anything the engine will almost stall out. When the engine is set up at 20 deg TDC, the car will run and accelerates up hills. The power is not huge but the engine is only supposed to have 90hp.

One of the "selling points" that the previous owner mentioned when we bought the car was that it had a new timing belt cover. I had no idea why he would tell me this, but I am beginning to believe that the old one was damaged when they changed the timing belt last. If you saw the vehicle, you would venture to say that the timing belt broke before they replaced it. I also have a friend who says that it could be the O2 sensors giving bad information to the engine management system. These are two completely different directions but it would be nice to know which direction to pursue first.