I have a Civic Exi December 2005 which was converted to CNG earlier this year. My experience has been phenomenal!
There is a convertion centre at Chauburgi called 'Inflex'. They use the screws of rear shock to fix the cylinder with minimum drilling in the boots. As per my experience, these guys are excellent!!
Price should be around 37K for a 55KG cylinder, though I am not certain!