my advice: use a electrical pump, easy to maintain and no hassle while starting in morning if u switched off car at cng. install a kabli one.
mechanical will always be problematic, bcuz when on cng, its output is blocked by a electrical valve which is installed for cng. the blocking by electrical valve creates pressure inside fuel pump and after sometime, it leaks.
i have personally checked it in a freind's mehran, and found it worth installing.