if it is a CG10 or 13 then there's really only two things on the engine which could really cause a power issue in 1st and 2nd gears AFAIK
one is the distributor which has an integrated Coil and cam (crank) angle sensor both which are prone to heat related failure on the CG10 and CG13. Replacement is typically the fix, a whole new distributor assembly. My experience with this is that when it begins to cause problems the engine will begin to miss and then eventually stall out and not be able to be restarted until it has cooled down.
two is the air flow meter which is integrated into the throttle body. These can sometimes be pulled apart and the circuit board re-soldered to resolve the issues but some do just need to be replaced. This is likely to be your problem. There is some coolant routing to the TB you will see around the back, i think these are here to prevent the butterfly from icing up in cold climates - you could probably block these coolant lines to reduce the temperature of the TB which might extend the lifespan of the circuit board.
The 02 sensor could be a possibility but I think it is used more for cruise emissions rather than acceleration. there's a few other temp sensors on the motor and a neutral and reverse position switch on the gearbox but i doubt any of these could cause you problems.