Akra won't sell the complete exhaust any more but they do sell repair parts. Took a bit of research to figure this out and find a vendor.

Couldn't find a vendor in the US, finally settled on Omnia Racing in Italy. Could probably have special ordered from SIP or Falan but Omnia had all the part numbers and prices online and internet reviews said they were like a Euro Revzilla.
https://www.omniaracing.net/

Order process went pretty smooth, no problems communicating in english. Took 6 weeks though as all had to be back ordered from Akra. Only issue was box was a bit beat up, could have been packaged better, can was encapsulated in styrofoam from Akra so it survived. Price was about the same as the newer version twin tip pipe. Freight from Italy was pretty steep but no sales tax or VAT.

Part number had changed several times over the years, I ordered parts from the latest version, S-VE3S07, screwed up though as I didn't note that this version doesn't have a removable baffle. If I had a do over would order S-VE3S05 parts to have that option.
https://www.akrapovic.com/en/moto/products/Vespa/GTS-Super-300-Sei-Giorni-Tech?brandId=39&modelId=283

In my search for a used GTS, Quasar motor was a must because I wanted to use this pipe. Was then disappointed to find it had been discontinued. Thought I'd post this up in case any one else still wants the OG pipe.