As in title. We could add electric motor + battery pack to any car and make mild hybrid. And also 100++ kg of battery in any form. And we will be proud of getting better mpg.
3-5-10 kW of motor power is the least amount to feel any gain.
But... 3 or 5 or 10 kW is made in primitive (hermetical bucket plus simple nozzle and filter) and easy to make charcoal gasifier. And energy to weight ratio is sky-rocking in comparison with any battery. Worst case is like an half of gasoline.
By very rough calculation - 25 liter bucket, will hold about 5 kg of charcoal. Or in energy - same as 3 to 4 liter of gasoline.
Or by using this
https://en.wikipedia.org/wiki/Energy_density - ic motor efficiency of 15% - same as 41 kg of lithium battery...
If ecu stays in closed loop - it won't need any adjustment till 30% of extra fuel - it will just decrease amount of fossil fuel.
And yes, this solution do have two problems.
1. You will have to make your own charcoal.
2. Not plug in charging.