There is probably at least one alternator/starter rebuild guy near where you are. It costs less than $100 to get it completely overhauled. I found one five miles away. Also check an over the road truck repair place. They will have someone too.
When I had a Perkins, I was looking at a one for one replacement by (and I am going to horribly not remember the exact name) Leece-Neville/Prestolite. It will be an easy swap.
Don't go overboard upgrading the amperage unless you are going after changing out the wiring. If memory serves, mine was a 60A and I planned on only going up to 75A had I ever gotten to that project before we sold it.
2000 Navigator 4200 Classic
(NOT a trawler)