Hello, since i got a nice crankset and also a cool suicide-type front derailleur i'm willing to install on my robur from 1956.. Another good reason to do is because they are matching the already equipped simplex parts.
By keeping the currentl axle, the small cog touch the frame as you can see in the video below.


Here the spindle and his measures.
https://i.ibb.co/wgLYXmG/Whats-App-I...t-20-58-20.jpg
https://i.ibb.co/nMjV0J7/Whats-App-I...20-58-20-1.jpg
https://i.ibb.co/t8CLV8n/Whats-App-I...20-58-20-2.jpg

may is an useless info but the brand is smextra

What size should i try to find?
Also, any help regarding the front derailleur setup? looks kinda strange since sometimes touch the crankset too while shifting.

What kind of BB? Italian?

Non drive side and bearing width is pretty standard. If 126 is too short, look for a 128, 130,132. The added width goes to the drive side.

Yeah, you should be able to measure both what you have and what you need.

you can add a 1-2mm spacer on the drive side bottom bracket cup

‘if the adjustable cup has exposed Threads beyond the lock ring.
my suggestion would be to remove the cotter pin, and slide the drive side arm to locate a good chain line, measure the distance from the arm outside to the spindle end and, that would be the additional length you are looking for. Bearing race lands all being equal betwixt. Test, don’t guess.