Yes, the travel of the 2021-2023 SID (35MM) C1 can be changed by replacing the air shaft.
11.4018.111.006 FORK SPRING DEBONAIR SHAFT - (INCLUDES AIR SHAFT AND BUMPERS) 100MM-29 (35MM) - SID C1 (2021-2023)
11.4018.111.007 FORK SPRING DEBONAIR SHAFT - (INCLUDES AIR SHAFT AND BUMPERS) 110MM-29 (35MM) - SID C1 (2021-2023)
11.4018.111.008 FORK SPRING DEBONAIR SHAFT - (INCLUDES AIR SHAFT AND BUMPERS) 120MM-29 (35MM) - SID C1 (2021-2023)
RockShox service kits and spare parts are best obtained through your local shop or dealer. See our dealer locator, below, to find a dealer near you. If no one is currently listed near you, then please search for local shops via your preferred web browser.
https://www.sram.com/en/dealer-locator
Manuals and additional information about RockShox and SRAM components can be found at our service site, linked below.
https://www.sram.com/en/service
Further questions?
Reach out to Rider Support by Asking A Question at the link below.
https://www.sram.com/en/rockshox/campaigns/rockshox-rider-support