Really, try some PB Blaster for a few days at least. It's raining in MI, what else are you going to do?

Use copper pipe, it will deform before you wreck the stem, and fit inside the stem beyond the expansion point, so you won't be working against yourself.

No heat, just PB, tap it to vibrate it in. You could use Coca Cola, too. That crap will eat away at most anything!

Post a few pictures of where you are at in this task. Which model Cinelli stem is it?