There are Fortran to C converters, however I am not sure whether they support the Fortran version WSJT-X is implemented with.

f2c - Wikipedia
FORTRAN to C Converter - Automatic Fortran to C conversion (quickreckoning.com)

Fortran compiler is not part of the official Android NDK, but it seems as if the NDK could be compiled relatively easily to include gfortran.

buffer51/android-gfortran: Android NDK toolchains with Fortran support (github.com)
Danilo's Tech Blog: How to build the gcc Fortran cross-compiler for Android (ARM and x86) (danilogiulianelli.blogspot.com)

Interestingly enough, modern browsers support WebAssembly WebAssembly. One may compile a C/C++ code into Web Assembly bytecode, which is then translated to native code by your browser and ran at the speed close to native. Web browsers support audio input / output, so it is technically feasible to recompile WSJT-X to run in the browser, although it would require quite a significant effort.

73, Vojtech OK1IAK