What are the actual hazard levels and frequencies relating to fire risk?
We are not aware of any statistics that specifically cover the frequency or intensity of fire outbreaks, caused by diesel lift trucks however, we can advise on some precautions you could take to minimise the risk.
One of the most effective to prevent fire outbreaks is through a strict discipline amongst operators to undertake pre-shift or daily checks. Doing this diligently should identify any oil or fuel leaks which, if not fixed, can cause fires (and other hazards). The Fork Lift Truck Association produces a Daily Checks Booklet to help ensure this important function is managed and it can be obtained, either as individual copies or in boxes of 50, here: Inspection Docs > Daily Checks.
There are a number of companies that have products available capable of detecting and suppressing outbreaks of fire, one such company is Fork Truck Controls (http://www.forktruckcontrol.com/forklift-safety/firetrack/).
Another company that specialises in converting trucks to be suitable for use in explosive atmospheres, is Pyroban (https://www.pyroban.com/) who really are specialists is fire/explosion prevention and fire suppression.