That is an OR spark ignition engine, probably from the 1940's, or very early '50s. I'm not sure of the displacement, but I think it's a .19, although it could be smaller. The displacement should be embossed on the front of the engine case, just below the top of the aluminum casting. The distributor is on the front of the crank-shaft & has a little lever that is used to advance & retard the spark timing. The "thing" in the top should be a spark plug, unless someone has done a conversion to glow ignition (that's what it looks like).

I suggest that you try to sell it to a collector -- you'll get a fair bit more than $5 for it, if it runs.