I took such a decision last year, and quite happy with it! Although CNG is a very good alternate, but not suitable for an engine not designed for it. Engine always run lean causing excessive wear and tear to the engine.
Regarding the checklist you have prepared, it seems good. Just add the following:
1. The entire connections of the engine controls (sensors, ECM, Distributor, CO) should be cleaned by contact cleaner first and then resprayed by WD-40 to avoid corrosion. Avoid spraying the contact cleaner on the sensors itself as these sprays are not good for them.
2. As you have mentioned that head is weak, it will be preferable to get the compression checked and replace the rings too if required. Also replace the water pump while changing the timing belt as well as get the oil pump replaced(if required). Get the genuine timing belt not the cheaper ones to be on a safe side.
3. If radiator is too old, chances are greater that it has choked tubes here and there. Do get it checked by your mechanic and replace if necessary.
4. I would advise you to change the throttle body too, as the CNG nozzle wont allow optimum performance and economy. Secondly, close the holes made for CNG cylinder by #1 rubber caps to avoid water entering the boot area.
5. If fuel pump is faulty, get a 'Bosch' motor as a replacement and use the same old housing (Hopefully the electrician must be aware that Korean motors have reverse polarity than other counterparts). Get the genuine fuel filter too and make sure it is not installed in reverse direction (which mechanics usually do). It has an arrow mark showing the correct direction of installation.
6. If going for plugs change, get the NGK BPR6EY ones.
7. Regarding the AC compressor, if it is not faulty, do not sell it as you wont be able to get a good kabli compressor in future.
8. After everything has been in order, get the main earth as well as main positive contacts cleaned too (Battery terminals, battery-starter +ve contact, starter-alternator contact, battery-starter main earth contact, battery-body earth contact, ECM earth contacts), so that you provide optimum current to the ignition system for a better spark hence better performance and economy.