Living in the Ireland, where they introduced an emissions based tax system since 1st July 2008, I'll make comment on what they do here, which may be no use whatsoever to the OP...
Any NEW car registered after 1st July 2008 will a) Be subject to Vehicle Registration Tax at a rate depending on the emissions band it is in x Open Market Selling Price (OMSP). This can be between 14% - 36% x what the Revenue office thinks the car is worth (and this has a "floor rate" (i.e. minimum value) of €2000).
The car is then subject to annual motor tax which also is dependent on the emissions band the car is in. (Band A=€104 to Band G=€2100)
If a car is registered in Ireland before 1st July 2008, the annual motor tax would be based on engine capicity in cc bands from sub-1000cc and in 100cc steps to over-3000cc. This would be in the range €172-€1556
The difference comes with how imports are treated, as "registration date" for these purposes is "the date of first registration
in Ireland."
If I import a pre-2008 car the VRT calculation would still be done on an CO2 basis, but the motor tax will be on a cc basis.
For example, if I were to import a 1982 Volvo 343DL Auto, the car would be subject to 36% VRT (based on the floor rate of €2000), which would be €720. Then it would be subject to annual motor tax of €333. A total of €1053

(The old VRT system was a mix of engine size and OMSP subject to a minimum charge of €315).
Logic would have suggested that pre-2008 cars would be subject in whole to the old arrangements based on the vehicle's first worldwide registration date. Sadly that was not to be the case. Fortunately on the bright side, we have a rolling 30 year-old vehicle concession that means vehicles over 30 years-old pay a fixed rate of €50 VRT regardless of value or engine capacity or emissions; annual motor tax of €46 and exemption from National Car Testing Scheme (MOT)
The Revenue charges the 36% rate to cars either with band G emissions OR no satisfactory evidence of emissions. If the vehicle was registered post-Mar 2001, the the UK V5C emissions figure is accepted. Go back a little bit further and you have patchy data and back a little further still and you have none. However, the Revenue takes on board quite a range of documentation found
HERE. Of interest is:
Irish Revenue wrote: Second-hand Vehicles manufactured prior to 1997
In certain instances the level of CO2 emissions may not be available for vehicles manufactured prior to 1997. In such instances, if details of the fuel consumption of the vehicle are available - the combined figure derived from an average of urban and extra-urban figures (obtained from any of the Revenue approved sources only), the level of CO2 for the vehicle may be calculated using the formulae below.
It should be noted that all calculations using the formulae below must be carried out by the declarant in advance of the presentation of the vehicle at a Vehicle Registration Office (VRO).
1. Metric Calculations:
1. where fuel consumption is shown as litres per 100km:
CO2 emissions = fuel consumption X 23.20
Example
If the consumption is shown as 5.8 l/100km then 5.8 X 23.20 = CO2 emissions of 134.56 or 135
I did have someone come to me trying to find emissions data for a 1998 Reliant Robin, as his local Revenue Vehicle Registration Office wanted to charge him €720 for the pleasure of bringing in a vehicle from Northern Ireland

. I sent him some mpg figures (unofficial manufacturer's data), but my main argument was that it wasn't a car, but a tricycle and no official fuel consumption figures existed because the vehicle type was never subject to fuel consumption tests. After appeal, they decided it wasn't an EU M1 type vehicle and calculated tax at the motorcycle rate which gave the owner a rebate of €600.
All this registration tax is a con. The Netherlands suffers from it too (BPM?). You really don't realise how cheap it is to run a motor car in the UK...
What the OP needs to try and do is lobby his local political representative to amend any mad CO2-tax bill to incorporate a workable scheme for older cars where the data doesn't exist, or move.