just hook up a scope and try figure out what's happening when you move the timbre knob.
as you'll see, the FM module is simply two sine oscillators, one modulating the frequency of the other.
That said, using the factory sine oscillator won't be the same as internally it doesn't have an fm-index-ratio that scales the modulation with based on it's internal frequency.
Though, if you go through my oscillators, you'll be able to find several modules that include both a fm input with fm-index internally calculated as well as sync input. So as long as you use a sine wave as basis for both oscillators, it will sound the same as the fm module of mutable instruments. But lots of my oscillators also allow you to morph the waveform so that you can create more complex sounds then just those two modulating sines.
The overfold of that trianglefold oscillator is done by just using a sineshaper over a triangle oscillator and adding gain to the triangle before the sineshaper (use a "multiplier" and "shift" module from the math folder). If you use the factory triangle oscillator together with a gain module and sine shaper, you'll get the same effect.
Though, the triangle oscillator doesn't have a sync input. Again, just use one of my oscillators, add a gain and sineshaper behind it and you'll have the same thing with more options.
Or why not do both MI oscillators in one? Basically the only modulation of the trianglefold module is that it's adding a sineshaper behind the oscillator, so you could do the same with any other oscillator.
so for a basic setup:
-sync oscillator (that gives the wave that syncs the other oscillators, just use a sine oscillator for this)
-any 2 oscillators that have a sync &fm input (both connected to the sync-oscillator) and some wave-morphing/selecting
-connect the output of one of the 2 oscillators to the fm input of the other
-add a mixer to be able to mix the oscillators
-add a gain and sine shaper behind it