It depends if it smokes all the time, or just on the overrun – when you close the throttle.
If all the time, it is burning a significant amount of oil, so if this isn’t topped up every few hundred miles, it will run out of oil and be wrecked.
If just on the overrun, it will need topping up less frequently.
Also, you are producing a high level of pollutants.
It is also unlikely to pass your yearly MOT test.
The piston, rings and probably cylinder bores are worn and need replacing