If you have a China girl, it will not be stamped anywhere on the engine.
You can use a caliper to measure bore and depth gauge or dial indicator to measure stroke (after removing the head).
Harbor Freight sells a "Technical Measuring" kit that costs around 10 bucks that will probably do it.
If it is a 2-stroke meant for motorizing a bike it is either 48cc or a 66cc, there is no such thing as an actual 80cc, that has been marketing crap for years.
Bore (cylinder with) x Stoke (how far the piston goes up and down).