Please read the ASA Guidleine 2b on modification of bikes.
https://asa.ch/it/biblioteca-online/direttive/
It is a must read for any wannabee bike modder as it explains the law and regulations. Some modification mus be shown, certified, and will lead to an entry in your gray card. The ASA is the assoiaction of the Strassenverkehrsamt. The very places where you have to bring the bike for inspection.
The welding part is the most tricky one as it must be done by a certified welder and can not be easily undone. Losing veteran status is not much of an issue, except the slightly higher tax and the 2 year inspection period.
Worst part is that the bike may lose value in long term. More and more "Cafe Racer" less and less Bikes in Original condition. If you keep the original parts and only do modification which can be reverted you have the best of both world. A collectible in the future and a hippster bike now.