"New" part number is 319253125 which superseded other one.
Eurospares now have in stock at £40 plus vat but on Ebay in States for about $18 but shipping costs are high.
Still looking for alternatives but this is the part in question.
https://www.maseratinet.com/ps-20437-32-fuel-filter-pre-pump.aspx
The inlet diameter is the problem as it is about 14mm. Would rather get the correct size than sleeve it down somehow.
Good news is tank is clean and almost free of rust. Been flushed several times and likely candidate is the filter being choked with this silt from the fuel as there was a fair bit in the tank before it was drained.
Other filters turned up ok so wont take long to reassemble once we get the missing filter.