Replacement part for MAX407 op amp
Rather than suggesting a part, I will look at the key specifications in the application.

The key requirements appear to be ultra high input impedance, unity gain stable and total supply voltage greater than 9V. The MAX407 has an input bias current (typical) of &lt; 100fA and is unity gain stable. It also has a low gain bandwidth product (8kHz typical) which may be important for stability.

A number of parts would do the job:

This parametric table lists a number of parts from Analog Devices.

Make sure the device can withstand a power source of 9V (many newer parts cannot).

TI has a rather interesting part (25fA Ib)

I am sure there are many other parts that would do the job.

Gain bandwidth product may be important in the sense of keeping it low; you will find those in the ultra low power selection tables (that all the manufacturers have now); these devices (designed for IoT and remote sensors in  many cases) also often have very low input bias current.

Select a part with input bias current &lt; 10pA (max), unity gain stable and Vs (max) > 9V.