Heat riser

Most likely, the internal plate is corroded away, so unlikely it is restricting the exhaust. Mine was stuck solid decades ago, so removed it and plugged the shaft holes w/ stainless bolts & nuts. I did this when the manifold was off. I sure wouldn't remove the manifolds just for this issue. Of course, try solvent first. If you can't get the correct solvent from a dealer, maybe try wood stove glass cleaner. It dissolves soot, which is probably the same problem. You can find at Ace. I lived mostly in the SE with my slant, but did have it 2 winters up North. I don't recall any problems starting or driving in the cold, and I think the riser was stuck then.