I wouldn't even try to do this. The furnace is past it average life span. Although I have seen many 20 year old furnaces that are still working as I am sure we all have. As far as the Lennox pulse furnaces this would not be a unit that I would sink $$$ into or for that matter any unit that old.
Take the buy out find you a good contractor and get a new furnace installed that has warranty. This way you want have to keep worrying about a 20 year furnace and if the warranty company installers did the job right.
I must say again that home warrantys are a joke when it comes to heating and cooling equipment, water heaters and electrical work. They will send out the lowest cost person to try and fix and or replace the system.