I had problems like these on My Nissan GTX and 1200 delux,
So lets try and help.

-Is your radiator clean, if not use some flush,

-Does this car cool down after going downhill on neutral gear, for a little while?

-Does she heat only on short runs?
-Does she heat if you stomp on the pedal for a long strectch of road?

-Does she heat up whilst idling?

-Does any of your water pipes has major bends on it, which is restricting water flow.

-Does the water in your radiator circulate faster as you increase revs.

BTW , the mazda 323s radiator is good enough for cooling the datto