Repowering with a Tractor Motor
IIRC, your riser is sch40, 316SS. You may ask him or a weld shop to clean, pickle, and passivate the riser. This would not only make it shiny again, but it would enhance the corrosion resistance of it, particularly the heat tint on the welds. Shouldn't take much time or cost much. Material is available at most weld shops, usually 20% nitric acid in a spray or gel, but citric acid is also commonly used.