I recently the bought a 20-3228-1 Big Boy as NOS and everything works great except for the fact that smoke comes out from everywhere except the stack!
I removed and dismantled the smoke unit to ensure that the stack wasn't blocked (it wasn't!) and found that the paper smoke reservoir gasket has crumbled to dust in several spots around the edges. This allows smoke to escape between the reservoir and the top circuit board.
I couldn't find any gaskets listed on the MTH service website so I'm hoping someone can tell me where I might find a replacement gasket.