you are mistaken in understanding the fundamentals of engine design - a low CR engine with a less dense slow burn fuel will result in poor performance as it cannot provide strong enough shoves to the crankshaft (low output). If you do want to change your engine to run on CNG only then firstly you need to optimise airflow into the engine and ensure that you can provide enough fuel (CNG) in it and also ensure that the engine can give good output - raise CR and optimise ignition timing too.
The engine designers made yours to run on petrol, you the user want to run it on CNG, that means its now your job to come up with a completely proper working, repeatable, documented and applied solution to get the same engine performance from CNG fuel in the same petrol engine.
Problem is that in Pakistan people see as the engine is "start" they automatically think - JOB DONE. Its not - its like an olympian champion sprint runner walking and a 95 yrs old person with arthritis and using a walker also walking - both people walking does not mean both are the same kind of person when it comes to performance.