I rebuilt the starboard engine this winter. Mercruiser 8.1s. I am looking at Sierra manifolds and riser. Freshwater boat. The boat is 15 years old. I only absolutely know it’s history for the last 3 years. It does have a known history of having been in saltwater for its first 3 years in Wilmington NC. After that I have no idea until I bought it 3 years ago. Do I replace both engines manifolds and risers? The set on my starboard engine pressure tested ok. The mechanic is still recommending replacing manifolds and risers due to age. Mercruiser wants stupid money for the set that is why I am considering Sierra. My fun money account is a bit drained at the moment. I am usually better safe than sorry but I expect to have to eventually rebuilt port side. Any input?