i have done something similar but not exactly like above.
what i have done through a mechanic is:
1-removed the venturi
2-de-advanced timing, optimized for petrol, got car retuned
Ran car several km's to check feul consumption on petrol
found that the difference in running cost on petrol vs cng was very little so decided to remove CNG tank (cylinder) as well and went to the same mechanic again after 3 days
3-removed CNG tank (with approx. 4KG CNG still inside), closed the VALVE of CYLINDER and placed the CNG Tank in my garage/car porch, filled the HOLES in car body with SILICONE/SEALANT.
I also need suggestion as if the Tank placed at car porch/garage shall be emptied??i suppose as a security precaution it shall be emptied and placed there?
PS the important part, the mechanic only charger PKR 100 (one hundred) for removing cng tank and fixing with sealant. Revits filled in for r.s. 20 (4 revits) for packing with sealant.
so wityh so little cost it is better to get it done through a professional, who knows how to do it better.