What type of engine do I have?

So I'm a high school student, in a shop class, i'm rebuilding a riding lawn mower, but needed an engine, the shop gave me a free small engine. My only problem is that i need to buy some extra parts for the engine, and for that i need to know what type of engine it is exactly.
This is what I know.
Its a one cylinder 4 stroke honda with over head valves and the head is not removable, which is odd to me. This is all i know about the engine, i have no model number i don't even know what the engine came out of.

Look for a serial or part number stamped on the engine somewhere and how many CCs it is, that should be enough info to go to your local honda lanmower dealer and ask for some info on your engine or look it up online so you can find a repair and parts manual

Thats just the issue though, if i had the part number i would do just that. But i've looked for the number and can't find it anywhere on the engine
i could probably calculate the cc's of it though, that would help, thanks.

Where would you buy parts? A Honda dealer? Take it there and ask them to identify it.