Low-pA TIA Design and Noise Floor
Looking at the low frequency voltage noise (0.1 to 10 Hz) from the AD549J and the LTC6268 I would say the AD549 is superior at 4 uVp-p. The LTC6268 has a noise of 13 uVp-p. So this is something to be wary of if you are considering a change.

For the input current noise at low frequencies, nothing appears to be stated that is much below 10 MHz for the LTC6268 and that would be a concern. The AD549 is 0.7 fA RMS from 0.1 to 10 Hz by comparison.

I think the LTC6268 is better suited for high speed photodiode applications and not low frequency applications like ion beam measurement.