Hi Jonas,
Many thanks for this complete answer - and very useful advises.
Indeed, given this article (in French), they say the "early century machines (including the 45K) should be able to work with a 2,5-3mm yarn".
But, that means - yes - finding size 300 needles, and I had not considered that it would cost that much.
https://maxandshed.wordpress.com/2020/12/23/une-singer-45k-par-mscr/
In top of that, I have checked with French historical supplier (of linen thread) and their thickest quality is a 1,2mm - so, quite not the 3mm I have been considering at first.
The idea of sweing the thread with another thread can work, but I am not sure that the tension of the linen yarn will correct in this case.
I think I will consider two options for now :
. Hand sewing with a thick yarn (3-4mm).
. Working with a 1,2mm thread, and here maybe the job can be done on a regular heavy-duty machine?
I will make some trials!
Many thanks again for these good words.
Clément