Century makes an LPG filter assembly that has a filter pad similar to the Impco plus it has a magnet in the housing that will collect the iron particles. I don't know the Part number at the moment but it is the same assembly that Daewoo/Doosan uses. It mounts at the rear of the truck like a bulkhead fitting. Tank hose and hydrostatic relief valve connect at the inlets, has outlet which is inside engine compartment. Comes with nut to mount unit once a hole is bored through sheet metal or light gauge steel.
6 screws hold assembly together so that filter pad can be changed and magnet can be cleaned. Filter kit comes with pad and o-ring for reassembly.

Carl, thanks for your prompt response. The impurity that worries me most is the iron filing from the internal walls of the tank.
In the past Hyundai forklift were equipped with the filter VF300 and we use to open the filter and regulator each 500h for cleaning, replacement of filter element and membranes. The new models have no longer this filter which should shorten the maintence intervals what we want to avoid.
As there is not much space close to the regulator to install the filter my major concern is the lost of pressure of the LPG and the freezing of fuel before reaching the regulator. What do you think?
Regards

What type of impurities are you refering to? If there are excessive tar or heavy ends accumlated in the regulator and mixer you might want to install an inline thermostat in the coolant return line between the regulator and the suction side of the water pump. This keeps the water temp down to about 140 deg F in the regulator and slows down the refining process of the LPG in the regulator. Filter assemblies generally catch scale and debris from the tank like iron filings.