I presume you/customer are wanting a "tail pipe" test kit?
Where I work we use a Blanke Industries CO meter.
I looked online before writing this and it appears that the exact model we use is not featured in their description.
But ours is the equivalent of the Model 1500 that is shown (in that it reads in the range from 0% to 10%).
The only difference is that the one we use is about 8 or 9 years old and DOES NOT feature a printer. It looks to be the same actual meter but without a printer.

Testing of CO emissions is a "2 pronged" situation (for want of a better description).
If OSHA or other authority performs a test or evaluation, it is most probable that the test sample will only be taken from the general atmosphere in the work areas, NOT from each vehicle exhaust pipe.
If the CO level exceeds the maximum permitted value, the employer must do whatever is needed to lower it to the permitted level.
BUT, the employer must be able to produce documentation proving that each IC engine in use in the work areas have been tested for CO emissions and are in compliance with the maximum standards allowed for tail pipe emissions coming from that specific vehicle (or engine).
So, it seems to me that a CO meter is essential for anybody working on IC industrial engines cause if you keep the tail pipe emissions of the vehicle in compliance.......the general atmosphere levels will be right too.
I have only been through 2 on site emissions tests where OSHA came in and tested and in each case ONLY the general atmosphere levels were sampled. No sampling was done on individual lift trucks.