Kamran Bhai, Sarmad AOA, hope all are well. I was going through your comments for the past few days which I was away hunting for FJ 40. Sarmad would know more as he has allot of experience in 151 and info, till now who ever I consulted for crab, every one suggested supper carry 1000cc. It seems that it works well with good output. I would suggest not to go for a brand new one yet, as it will be costly. You should be able to get a supper carry one for around Rs. 2500 to RS. 3000.
Make sure that you keep the original one with you. Job on your jeep been done very swiftly, looks nice, but as suggested by Sarmad, keep the originality intact. Good Luck, unfortunatly I am out of 151 for the time. I managed to get 84 model FJ 40, start condition.