Just For A Sake Of One Mount.....You Want To Shift To Be On E Series....4EFE And 5EFE However Are Rear + Comparing To 5AFE 4AFE....Parts Are Rear Too... 
I Still Say A Series (H)
@irfan...!!
Bhai 20v is Not A joke....You Wanna Swap It For What??? For Economy..Then Its Not Made For It...Power..No..On CNG..Will Loose Power.....!!
I Myself Have 20v BT....Just Feed It With HOBC And It Will Give You True Power Pleasure...!!
Drive less But Get Pure Power.....Other Than That...4AFE 5AFE 7AFE Is There To Serve You On CNG Quiet Well