Choosing a non-cryptographic hash function for language with no unsigned integers
The offset is unlikely to matter.  As the FNV web page says:


  almost any offset_basis will serve so long as it is non-zero


The multiplication is potentially more important.  The FNV-1 hash is defined to use unsigned multiplication modulo $2^{32}$, but if UnrealScript only has signed 32-bit integers, presumably you have no way to do that kind of unsigned multiplication.  At that point you're no longer using the official FNV-1 hash; you're using something different.  I suspect it's still fine, though.