The gicleur is cheap to buy and easily available…It is available in various sizes, for yours it depends on the pin size. A usual size is 7mm, but it could be 6, you need to accurately measure the diameter of the pin or see if it’s printed on the top of the gicleur.
They are only brass and definitely don’t need to be screwed in very tightly….
P.S. You could ask Lelit and they will probably happily give you the jet size you need to order
