Is the xxh64 hash suitable for file integrity checks? I have a game and inside this game I want to check some files integrity before loading (1mb+) to battle cheats. I know that this xxh64 is not suitable for crypto, but is it suitable for my purpose? So for instance if I have some file and I know this file exact size and xxh64 hash, how easy is to create fake file of same exact size and xxh64 hash? And even if I find collision file with same size and xxh64 hash it will be completely garbage, not suitable to use inside game (like texture, sound, animation etc.).
One of the main features of a cryptographically secure hash function -- probably the main feature -- is "preimage resistance", the property of making it computationally infeasible to find an input with a given hash. Preimage resistance is basically what you're looking for, and it is not a feature of xxHash.
More details on xxHash's weakness to preimaging are given in this question. But the short answer: No, it's unsuitable. (It's also unnecessary. Modern processors are crazy fast at computing, say, SHA-256. There is no possible way that that will be the bottleneck of your gigantic data.)