most of the stuff that operator synth does looks fairly straight-forward, shouldn't be too hard to put together a basic pd clone.
challenging bits will be the square and saw waves with variable numbers of partials, but i think that will probably just be a case of [tabosc4~] and making some tables with sinesum values.
then that knob to alter the amount of aliasing, it could be using upsampling and then lowpass filtering like the nord synths do, or like the 'classic synth' example in pd's audio documentation. but that approach quickly eats up cpu....or else it could be using an adjustable bandlimiter, something like the [blosc~ comparator] external, but with adjustability. actually, this function is the hardest one for me to think of a solution for.
the fm and envelope stuff would be super simple. just the quantize thing woudl take a bit of fiddling, but can be done with totally vanilla pd objects.
an audio rate lfo would be super simple (easier than control rate in pd)
the filters shouldn't be TOO hard, although to get a good lowpass, notch, bandpass and highpass it might be best to dig through pd-extended's externals. for lowpass i usually use [resofilt~] bandpass vanilla's [vcf~] is fine, although franks new resonz~ / resonr~ filters look super promising. for highpass, i prefer [svf~ high]. not sure about the notch filter.
there are also a lot of fine details that would be easy to implement, which shouldn't be overlooked...like the velocity to filter envelope mapping (sounded good) and that fm automation sync with quantize on/off. then that time knob, and some of the other squinty stuff in the bottom right hand corner.
actually does look like quite an interesting project.