Keeprite NG furnace code "fail to ignite"

When you swapped-out the stat, have you ever had heat since then?

The old T87 stats were indeed single stage, and your furnace can run with a single stage stat just fine, as it has internal logic to change from Low to High, as long as it was set-up properly initially.

When you changed the stat, did you change any wiring, or DIP switches, in the furnace?