The main critical part in the µCurrent is the max4239 AZ-OPamp with a good bandwidth and low offset. Everything else should be easy to replace with different parts / manufacturers.
For the resistors one has to see what is available at a reasonable price. The search functions at the suppliers should help here.
For the µCurrent there is sometimes the misconception it could be a good solution for small currents. For the really small currents (e.g. < 10 nA) the better solution is often a transimpedance amplifier configuration and using a more normal (non AZ) amplifier.
IFAIK the lower current ranges have no protection with clamping diodes - I would add that option to a PCB if I would build a version.
Ideally one may want more shunts (e.g. 1:10 or at least 1:100 steps) to choose from. The current solution is OK for some currents, but not for all. The 1:1000 steps are just to large to cover everything in between. It would also make sense to have a case with no shunt and thus the option to use it as an amplifer for low voltages (e.g. a thermocouple).
The nice point is having DC accurate gain with low offset. This may help to add low burden current ranges to a DMM, but is less important when used with a scope. With a scope one has a DC offset from the sope anyway and noise is often more relevant the the offset.
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