Hydraulic disc brakes on cross bike (post mount)
It sounds like your post-mount frame (and fork) is setup by default for 140mm rotors.   This would be expected for the frame and is not surprising for the fork either.

What you need is a post-to-post 140 to 160mm adapter.  This might be more commonly a 160-to-180 adapter, but it's the same thing.  It's moving the caliper ~10mm further away (or more like 12.5mm on top and 7.5mm at the bottom).



Shimano makes the SM-MA-F180 P/P2.

SRAM makes the "20S/20P" adapter also for this purpose.  (This is the one I have used.)  From SRAM's instructions publication, there is a handy table that illustrates that going from 140 to 160 uses same adapter as 160 to 180.



You might want to first check with a local shop to see if they just have the spacers you need in a bin.  Otherwise you can definitely find it online (e.g. at jensonusa, but obviously it can be found elsewhere too).