I have a spare exhaust too - never used it. Beta had two versions on the elbow and I wasn't sure the standard one would fit the engine space so they provided a second, shorter one that I never used. The one fitted is stainless, the shorter is cast aluminum.

As for spares. Zincs and O rings for the heat exchanger. Impellers of course. I always carry a few spare belts. My second alternator eats one every couple of years. Beta had a spares kit as an option with the engine. Did you get one of those? It had a good start on spares. I buy six oil filters and fuel filters at a time so I always have them available.

I am trying to think of issues with the engine and can't think of much. I did modify one of their custom brackets but that shouldn't be a factor for you. I did not like their copper piping in close proximity to the aluminum transmission cooler. When I replaced the transmission and cooler (I had kept the original when repowering) about five years after the install I removed all the copper and replaced it with radiator hose. The copper was still good but it caused electrolysis on the aluminum hose barbs on the cooler.

I did replace the injectors after about 2500 hours, long after the recommended interval. I've checked valve clearances a few times but they needed very little adjustment. Other than that it has just been standard maintenance. Just before the warranty expired I had a Beta factory authorized mechanic check out the engine - no issues. Other than that I've done all my own maintenance.

I need to go to Port Townsend soon for Okume plywood. I'd like to see you and your boat. When are you usually there?