The FFT function in REAPER only supports sizes up to 32768 (complex pairs).
For longer FFTs (such as required by PadSynth), you can calculate the FFT as a combination of shorter FFTs (Cooley-Tukey factorisation).
You can either include it, or copy the functions from fft_big.jsfx into your code.
fft_big(block, N, working_space);
ifft_big(block, N, working_space);
The results of these functions are in order - that is, there is no fft_big_permute() function because that's already included in fft_big().
The implementation (currently) requires some free memory to use for intermediate values, which is passed in as the working_space parameter. This working space needs to be 256 slots long.
Do whatever you want with this code, just don't blame me. :) Bug reports appreciated.