Aren't micro reactors meant to be like one-use batteries? That's the info I got from listening to the Omega Tau podcast about this.
The expensive part of nuclear power plants is all the electricity generation part (turbines/generators/piping/transformers/etc) that is pretty much the same as for coal power plants. You don’t swap out any of that.
I guess if the supplier goes out of business, you might have trouble, but I'd expect these companies are putting things in place for that eventuality e.g. insurance.