hello!!
1st of all its MPFI system is Santro Plus and Efi in Santro Club... u did'nt mention which santro are u talking about but as u say its MPPI so i think its Santro Plus with MPFi...
2ndly you ask about installing a CNG kit in it so definately the kits for injector cars are available in market...so it can be installed in your car,.......but becareful take the genuine kit with warrenty and get it installed it with good conversion centre....
Thanxxx