I've been searching the site - but I need some confirmation. My 1975 1.8 Non-appearance did not come with an Oil Temp Gauge.

Can I add a Tuna can set up and sending unit ? I think there should be a wire wrapped up under the battery support?

For CHT I was going to get a spark plug Ring
For Oil Pressure - get an adapter and sending unit to also keep dummy light

You are confusing a couple of our "nicknames"
The Tuna can is a mini-sump that mounts at the center botom of your case & extendes the oil PU into a resevior of oil to prevent starvation during hard cornering.
The TACO PLATE is the nickname for the Oil tempeture sender on the bottom-front (left) of the case that sends the signal to a guage in console or combo gauge.
Called a "Taco Plate" because the OE part was made from a thin stamped steel that commonly gets over tightened (into the shape of a Taco) to eliminate an oil leak only to make it worse!
Yes. A green w/red strip wire should be present eminating from the main loom just under the battery tray.

Sorry about that !! Too many T’s. So can I add a taco plate and install a sender unit ?

Absolutely. First check that you dont already have one installed
If not you will have a blank plate in its place.
Im biased, so Id consider the billet unit I offer over the OE piece unless you want to keep it all original.