The first rule of thumb in the marine world is never trust old guages, they give you a general idea that is about it. I have seen tachs on a twin engine boat side by side saying 4k rpm and 3.7k rpm, but in reality the engines were turning 3,840.
Just because your 20 year old temp guage says 100C doesn't mean your engine is running at 100C. You need to check the accuricy of the guage before spending tons of money. For all you know the guage could just read 10C high and you may not have a problem at all.