Neither of the N's.. ( 9 / 2 /8 ) had temp gauges. They do have thermostats that fit in the top radiator hose though. The cooling system for these engines is way over rated.. and will cool the engines very well, if in good working order.
The 8n had a pressurized system, while I believe the 9n and early 2n's didn't...
If you have a replacement radiator though, the cap will have been changed too.. The early non-psi ones had different locking lugs.
I've heard of people drilling and tapping for them, but havn't done it myself.
I'm not sure what size fitting the sending unit for the gauge is.. but i soppose if you didnt want to drill and tap you could remove the drain petcock on the side of the engine near the starter and install the sender.. or a t, and the sender and the drain plug...
Bottom line.. if you are boiling out coolant, clean that radiator inside and out, and change your coolant and the thermostat and see if that helps any.
Soundguy