It's an awfully simple cooling system, so I wouldn't worry too much about that. If it ever runs hot again you can install the manual thermostat bypass valve that Universal actually recommended for these engines. You can get a look at one
here at the Moyer Marine Atomic 4 site. You can get all the parts at your local hardware store, but this gives a good picture of what it looks like installed.
I put one on my boat because I have the opposite problem and run a bit hot. I have yet to flush the cooling system, but it's on my list of things to do for this spring.
Good luck!