Issue with `whitespace_after_*` not fixable

I have to transform (using a libcst.CSTTransformer) some 3rd party code, which might contain statements like `assert("foo") == "foo"`.
Although this is a valid python code, I get the error `libcst._nodes.base.CSTValidationError: Must have at least one space after 'assert'.`
I cannot fix this in the `leave_Assert` hook, as it is called after validation, and I cannot fix it in the `visit_Assert` hook, as the object `node` is frozen.
Currently, the only option I can think of is to do a regex search and replace before LibCST to add the missing whitespace and then feed it to my transformer.
Is there any easier way to do this, preferably within LibCST?