The oil filter used on Nortons has metric threads of 16mm X
1.5mm pitch. The filter is about 3" in diameter and 3 3/8" tall. One of its
original applications was a Simca car (French) 1963-1972. It was also
used on Citroens 2CV, Dyane 4 & 6, Matra, and Peugeot.

There is much discussion about replacement filters and brands on the web.

For simplicities sake the following Wix filter should work with little or no problem:

WIX 51032

Click here to find a local dealer.

Two sites we found to purchase off of the web:

Fleet Oil