Getting a second opinion on a possible cracked heat exchanger?

I had someone out for maintenance of my 25yo gas furnace a couple weeks ago, and he said that there's early evidence of a crack forming in the heat exchanger, since the flame flickers as the blower turns on (though it didn't sound like he used an inspection camera to check directly). No CO detected right now, and he said it might be okay for months or years, but it's impossible to know. Since it's so old, he said they might not even be able to get the replacement part, and the part plus installation could be multiple thousands of dollars. (EDIT: umm... [$292, in stock](https://www.repairclinic.com/PartDetail/Heat-Exchanger/AS-61579-09/2639162))

This seemed like a plausible issue given the age, so I'd started getting replacement quotes, but now I'm wondering if I should get a second opinion. I know with the age, replacement is on the near horizon regardless, but I already have multiple expensive repairs coming up and had really hoped to wait on this one.

What would be a good way to go about that--schedule routine maintenance with another company and see what they tell me, or explicitly say I want a second opinion on a possible issue? Any other insights are also welcome. The furnace is a Rheem RGPH-05EAUER.