Old Quadras have little info on the engine itself.

On the later ones, the flywheel has a big "QUADRA Starter" label.

Most quadras have a serial number etched into the bottom. This will usually be done by hand. Mine is "42945" the "42" identifies it as a 42cc. There may be a 40xxx for the older 40cc models.

Q25,35,40,42 engines are very similar:

Cup mount
Rear exhaust
Steel can muffler.
Spark plug angled to rear of engine.
Magneto/flywheel ignition.

An almost identical engine is the US engines. These useually have a label on the side of the Cylinder.

If you can post a picture of the engine. We might identify it,