I don’t have MMA8451Q on hand so I couldn’t validate. But by looking at the datasheet, everything seemed to be identical except for the data register layout. It appears that the 8 bit mode (low res) is identical and the high resolution mode needs only slight adjustment:

void getAccXYZ(int *x, int *y, int *z, bool isHighRes=true)
{
byte buf[6];

if (isHighRes) {
regRead(REG_OUT_X_MSB, buf, 6);
*x = buf[0] < < 6 | buf[1] >> 2 & 0x3F;
*y = buf[2] < < 6 | buf[3] >> 2 & 0x3F;
*z = buf[4] < < 6 | buf[5] >> 2 & 0x3F;
}
else {
regRead(REG_OUT_X_MSB, buf, 3);
*x = buf[0] < < 2;
*y = buf[1] << 2;
*z = buf[2] << 2;
}

if (*x > 8191) *x = *x – 16384;
if (*y > 8191) *y = *y – 16384 ;
if (*z > 8191) *z = *z – 16384;
}

But again, I don’t have the chip to test. But feel free to update me on how it goes.