How powerful is a 3.5 cc engine (model making)?

First of all, it should be noted that the small model construction motors are usually incandescent lighters. In the meantime, there are relatively small Benin motors a 3.5 ccm but rather is always an incandescent lighter.

The engine power of these engines is extremely different. Simple engines should have a good half hp or just under 500 watt racing motors around the 2 hp. In order to achieve such high performances, the motors also rotate extremely high, so around the 45,000 rpm. The actually achievable performance in an RC-Car depends on so many factors that the manufacturers of really powerful engines usually do not specify the absolute performance.

Then you ask for the “average” speed. Since the average of the speed traveled depends to a much higher degree on the route and the driver than on the engine, I assume that you actually mean something like the maximum speed possible.

This in turn depends very strongly on the vehicle type. If it is a Buggy, Truggy or Monster Truck, it has a 2speed geared etc.

A 1:8 smooth track burner can become significantly more than 100 km/h fast.