Here's what I did
Go to a marine hardware store or an industrial heating and refrigeration supplier. Buy a piece of ceramic fiber blanket roughly 1" thick. You can cut and wrap this material around your new pipe for a custom fit. Also, buy some heat wrap tape (a cloth like material) to wrap over the blanket and hold it all together. Shouldn't cost you more than $10-$15 total. I used black iron pipe as a replacement per a diesel mechanic's experience / recommendation. Schedule 80 added more thickness than the original, should last for many years. Be sure to insulate well before running the engine as the heat from this pipe can melt stuff, start wood on fire if the pipe is close to other equipment, bulk heads, etc. Good luck!!