What you found was, indeed, an intake preheater or "heat riser". It directs exhaust through a channel in the intake manifold (not into the intake tract) to heat the choke pulloff elements, which reduce the amount of enrichment as the engine warms up.
One thing that occurred to me is that you could have a chipped or burned valve in the engine. I'd perform a compression check, just to be sure. If compression is very low on one or more cylinders, check the valve adjustment. If it's correct, there is likely an internal engine problem. If compression is OK, and it still runs poorly after you get it back together, I'd get a Weber kit if you can shell out the cash.