Check out Systron Donner, in Concord, CA.
Might be higher cost, than some hobby type parts.
They make and sell FIRE and overheat sensors and the systems associated. Often 28 VDC for aircraft systems.
The company has sold two types of mechanical / gas pressure actuated sensors, both for 'Fire' (extreme heat) and also for 'gross overheat', a general condition inside an engine cowling etc. (Circa 1980).
Using Google, I tried to find the optical sensor products, inventor Clark Bright assigned patent to Systron Donner. But pretty sure they were using interrupted or 'chopped' analog from the low output level sensor. That way, it's an AC signal; easier to process and amplify.
Try not to violate any patents or copyrighted stuff (I did used to work there!)
I think maybe spend some time, Google Patents, to check out (any) other similar flame sensing patents.