Hi Keith
I'm afraid that there are no readily available blocks which implements Platinum RTD models.
My suggestion would be to quickly build it up for scratch using one of two workflows:
1) implementing the Callendar-Van Dusen Equation using Simscape Physical Signal library, and feed it to a variable resistor, something akin this (just a quick sketch, from a temperature sensor to a set of calculation into a variable resistor):
1) implementing it as a block using Simscape Language, starting maybe from a copy of the source code of the Variable Resistor in the Simscape Foundation Library:
Simscape Language is more powerful and elegant but maybe more difficult with 0 experience. Take a look at this example in the documentation if needed!
Getting Started:
Step-by-step of modeling a linear resistor with Simscape Language: