Ha, I finally found on trojansewing.co.uk, some parts that might fit,
for the shaft
https://www.trojansewing.co.uk/product/singer-29k-handwheel-shaft-82008/
for the gear, there seem to be two options :
https://www.trojansewing.co.uk/product/81869-singer-29k-handwheel-shaft-gear-81869/
https://www.trojansewing.co.uk/product/singer-29k-handwheel-top-shaft-gear-81872/
I sent them a message to get their advice.