When were talking about a "Furnace Board" we are fundamentally discussing much more then just a fan relay and some timing logic. We are also talking about the Flame Safe Guard control portion of the board.
I am uncertain on the changes regarding the UL, the AGA and the Manufacturers when it comes to a residential appliance and their integral FSG, but would find it surprising as to if they actually support a rebuilt control.
For those of us that dealt with the rebuilt FSG's when they were allowed could tell you all the horror stories that followed.
I would love to know the official response from the AGA and UL on this matter.