That's very unusual; I thought all 914s came with an oil temp gauge, either in the fuel combo gauge above or in the center console as part of the Appearance Group; maybe not? You have the Appearance group fuel combo.
I suspect someone replaced that combo gauge in the past and did not install the correct one. Slide out the gauge (use a butter knife or similar to pry it back out of the seal) and you'll probably find a green/black wire hanging out back there, unattached. Then, look under the bottom of the engine and see if you have another wire going into a sensor toward the front of the engine, the plate with the bump on it. Inside that plate is the oil temp sensor.
If the wire is still going in there, then maybe it's all still connected and all you need to do is replace the fuel combo with one that has the oil temp gauge. Those come up for sale all the time and you'll want the "silver dot" version to match the ones you have now.
http://www.p914.com/p914_gauges_combo.htmIf there's no wire going into the bottom of the engine, then you'll need to find where that wire is and reconnect it to the sensor. You almost assuredly have a sensor in there, as it is was removed the oil would drain out.
Alternatively, maybe your car had the Appearance Package and someone removed the center console gauges without replacing the fuel combo. That's less likely, however.
Time for some sleuthing, good luck!
Edit: nope, appears the 1973 1.7 without the Appearance Group did not come with a combo, per the page above,
QUOTE
...the fuel level / oil temperature combination gauge was introduced, but came only in 2.0L vehicles ordered without the optional "Appearance Group." For 1973 1.7L cars and 2.0L cars with the "Appearance Group," a standard fuel gauge (part number 914 641 102 40) was used.
So I'm guessing you probably do not have the sensor under the engine. But I'm also guessing that they used the same wiring harness for all cars so the wiring may be hanging out there somewhere...
Edit2: By the way, you can easily sand and reshoot that dash gauge surround. Each gauge will slide out of the rubber holding it in, then you can remove the rubber out, and then the gauges will fall back through the hole in the surround. The surround is held in by six Phillips head screws, easy to remove. Give it some sandpaper and ScotchBrite and rattle-can it with some flat-black enamel, install in reverse, it'll look as good as new.