From a parts perspective...and this is by no way/shape/form complete...
Marinized alt - not available so you're going to have to have one custom made. Not insurmountable BUT that's going to add cost to this.
Marinized starter - not available so you're going to have to have one custom made. Again $$$
Camshaft - I don't see a truck/camper grind available anywhere I look. That's what you need, not a high screaming street grind. So...custom. I have had (1) cam ground to a custom grind...it's in my 78 TA. Custom profile from R&R Engineering (Pontiac Engine Builder/Racer) and completed by Comp Cams. I don't see them even listing a VW part. You may be up a creek right there.
And then you haven't even dealt with marrying the block to the outdrive - be it Volvo or Merc.
Given that you're set on going ahead with the project, and there are cracks on the outside of the boat, it would be wise to table your engine swap until you determine the seaworthiness of the hull. My gut's telling me you have a very large fiberglass project before you even start dreaming about engine work. I'm betting that if you spend 5 minutes checking, you'll find the stringers and transom suspect at best, non-existent at worst.
Running the boat without those, you might as well chuck the VW motor directly into the water as that's where it will be when the hull fails. Not if, when.
Not being a naysayer, just a reality check. What you can get away with in a car/hotrod often does not translate to the marine industry.
