If you’re looking to adapt a modern water temp sender with a 1/8″ NPT thread to the stock Honda thermostat housing, you’re going to need an adapter. Here’s one in aluminium:

Part number AF912-M16-02S:

It comes with a crush washer:

A closeup:

Installed:

Temp sender installed:

This particular (inexpensive eBay item) temperature sender didn’t seat all the way in;

Make sure not to over-torque it as the cheap eBay senders have quite thin metal and shear off easily, rendering them often un-removable from the adapter. Just ask how we know…
This one is Australian made. They’re available on eBay – though make sure the items you’re looking at match the threads that you require.