It would appear that the ctrl and left shift keys are defective
There is a thread (which has been locked by MS!) here https://answers.microsoft.com/en-us/surface/forum/all/surface-book-key-replacement/9b528b00-79ea-41a6-92a3-2a05e2621f93?auth=1&page=6
One of the peeps on that thread has kindly offered a 3D model of the ctrl key for free and can be found here:
https://www.thingiverse.com/thing:2905308
Some of the testimonies from the thread say that it works quite well.