as has been noted, Pulse furnaces are no longer supported by Lennox. What parts are in stock, if any, is all that exists. If you do need a part and it is on the open market, it will cost you!!!
I would not retrofit anything into a Pulse, even if I got it at wholesale.
Have HX pressure tested and use the money you are willing to put into it along with the credit Lennox gives towards a Pulse replacement and get a new furnace.