That's not easy and you need quite a bit more information. The ppm of CO is a ratio of micromoles of CO per mole of exhaust
You need to know the total exhaust amount of the engine i moles per hour (or kilograms per hour and average molecular weight. From that and the ppm, you can determine moles of CO per hour. Use the molar mass of CO, 28 g/mole, to convert to grams per hour.
You also need to know the horsepower produced by the engine during the period in which the above exhaust was generated. Divide the grams per hour by horsepower.
Note that the figure will change at other operating conditions.