dc-dc step down converter model and the command of a JFET in LTspice
Making a model is not easy work. Your interest should be the behaviour of the model, not the exact, nitty-gritty details of its internals. That's not to say that you can't go that way, there are people who made transistor-level 741s, and more, but those models will also be very slow in terms of simulation time.
Therefore, as AJN mentions, the converter can be replaced by a voltage source, referenced to the local ground. IXDD509 seems to be an IXYS component. And the opto-coupler, is an opto-coupler. Unless there are specifics about it, a diode and a current sensor should do (or even some voltge/current source, for even more simplicity).