Schottky diode equivalence in RF wattmeter?
First, replacing this diode will require replacing both D1 and D2 to try to keep the diode characteristics matched. Trying to match an easily available Schottky diode requires the following points: 100V reverse blocking, 100mA continuous forward current, and junction capacitance at bias (1.2pF nominal at 10V reverse). Searching Digikey gives the BAT41 (product page). It matches the blocking voltage, forward current, and capacitance (figure 5) characteristics of the original diode. A cheaper diode with more robust ratings and a lower price is the BAV20,113 (product page).

Given the age of the original diode, reverse leakage and forward voltage drop characteristics will probably meet or exceed the original diode. Since your instrument will likely require calibration after rework, you should be able to accommodate more subtle changes in the circuit performance. If you're using it for mostly qualitative or ratiometric purposes, then your concerns about calibration are less and matching each branch is more important (and, with a complete change in diode, at least the same model of diode is required).