There is no replacement option from Microsoft, no warranty...
Try 3D printing:
Here is the 3D model for a spare Surface Book Ctrl key. The perfect solution to replace a broken key that Microsoft doesn’t provide as spare part.
Thanks @lequanganhpiano for sharing the 3D model for free.
https://www.thingiverse.com/thing:2905308
Use some 3D print service for SLA or DLP technology, it must be flexible or tough resin. Because the key has very tiny latch, it requires the best printer.