Reverse UInt64 with bits width of groups. Reverse bits (bits = 1), reverse half-bytes (bits = 4, same as reverse hexadecimal string value).

ReverseUInt64.cs

private static UInt64 ReverseUInt64(UInt64 x, int bits = 1)
{
    UInt64 b = 0;
    UInt64 mask = 0;
    for (int i = 0; i < bits; i++)
    {
        mask = (mask << 1) | 0x1;
    }
    for (int i = 0; i < 64 / bits; i++)
    {
        b = ((b << bits) | (x & mask));
        x = (x >> bits);
    }       
    return b;
}