Rebuild of my GS160 engine

I have a 1964 GS160 Vespa with an engine that needs a total rebuild.

It has a 4th oversize Piston in it now (58.8 size) and want to know if there is an option to go to 5th oversize, or can these barrels be Sleeved and go back to an original size Piston again.

I know these engines can go to a 180 Barrel, Head and Piston but rather not as I want to keep it as original as possible.

Any help would be appreciated.