Yeah, the Perkins should operate at 180. There are good reasons why the temp. shouldn't stay below 180. I totally agree that you should check the gauge with an IR temp. gun. Then replace the bad thermostat.
If it's been several years, then the antifreeze should be changed as well, while you have the system open.
Then I'd recommend a full power run to ensure the heat exchanger can handle the heat at full load. Heat exchangers foul over time, and you don't want to discover you have a problem when you're trying to outrun weather.
The temp shouldn't exceed about 195 at full load. Some increase from idle to full load is normal - the thermostat needs about ten degrees increase to go to fully open.
I'm amused by the folks who will spend tens of thousands on a boat and then invent all sorts of excuses to avoid spending a hundred bucks on clean antifreeze.
go figure
Good Luck!
JS