I need your help in this regard

I bought a 4m40 Intercooler 97 3 door Pajero a few days back, It had been converted by a previous owner to a petrol 3,500 v6 DOHC engine, After using it for a few days there is a loud TIK TIK sound coming from the engine, mechanic says that "Bigen phir gaya hai" I have no idea what that is. Also he advised me to convert the vehicle back to Diesal. Its my first time to buy any jeep.

your advice regarding above vehicle will be highly appreciated.

1. Should I get a new diesal for the vehicle if yes which one would be better
2. Get the current engine repaired
3. Also would a petrol engine be better then diesal if yes which one.