This may sound crazy but... What about a cheap dedicated video camera? They are tiny and could be put right in the burner area looking at the flame. Also a thermal switch with a temp probe (about $5 on eBay) could be used to tukrn on an indicator diode. Wouldn't be much harder than trying to replace the current system.
"Not all who are laundering are washed" say Bill & Heidi
'78 Excella 500,"The Silver Pullit". vacuum over hydraulic disc brakes, center bath, rear twin. '67 Travelall 1200 B 4X4 WBCCI 3737