heat exchanger 2gm20f

Hi there, anyone knows what this thing is and what it does. Going to change out the exhaust outlet and mixing elbow this winter, but this thing in the cirle coesnt look super great either and would like to do something about it, but .... have no idea what it is. :)

I removed the heat exchanger from a Yanmar 2GM20F and would like to replace the shroud surrounding the core. Does anyone if that is doable and if so, how are the brass end caps which the tubes pass through removed? Did Yanmar pressure fit them on or will a little PB blaster and firm tapping...