Lennox G8-137T-1A

I am looking to replace the heat exchange on the Lennox G8-137T-1A. Does anyone know where I may find one? Barring not finding one, I am trying to figure out how I am going to replace it as nobody seems to be much help. I have the G8, which for years the single attached garage converted into a rec room was always cold. It only has one 9" duct to it. I sped up the fan 20% by way of changing the pulleys & gave it more intake airflow. Now it is the warmest room in the house. As much as I like the G8 I know its a dinosaur, but I would rather fix it than buy a new one as I know it works.
I've had 3 furnace companies come in for estimates, (about 5 years ago). They all assured me that what they had quoted would move the air adequately. When asked if they would put that in writing, not one of them would. In short, I'm worried that a new DC direct drive fan won't move the air enough to heat the rec room. I should mention that I have an electronic filter system & the furnace runs 24-7-365 so I have no need for variable speed bla bla bla. Does anyone have any recommendations as to what I can do to figure out what is required to ensure heat to that room? The main floor is 1600 sq/ft with 1200 ft basement. The 9" duct for the rec room is 5 ft long off of a main duct 30 ft from the furnace. Any and all help would be appreciated.