I have mistakenly deleted the default "app engine service account" for my project - {id}@appspot.gserviceaccount.com Is there a way to recover it without resetting/recreating my project on Google Cloud Platform?
Unfortunately at this time, there is no way to recover the default App Engine service account. The solution is as you say creating a new project and redeploying your code there.
Should you wish to see this deletion prevented or default service account recreation made possible in the future, I strongly urge you to file a feature request on the App Engine public issue tracker while providing this example as a business case.
UPDATE: As filed by the OP, a feature request now exists on the App Engine public issue tracker as Issue 13085. Please star this issue to receive updates on its progress.
UPDATE 2: As part of the App Engine Admin API, one can now use the apps.repair API to attempt to address issues of default Cloud Storage buckets and App Engine service accounts. I would recommend trying this API before creating a new project and redeploying. I'm leaving this as an addendum and not the primary solution as it's not guaranteed to solve the issue.
UPDATE 3 It is now possible to undelete service accounts. As per the Undeleting a service account documentation, a service account may be restored if:
The service account was deleted less than 30 days ago
and
There is no existing service account with the same name as the deleted service account.