How do I know which head I have?

I left my disasembled engine with a mechanic that said it was not worth fixing. I bought a new used engine off the internet, with head on it. I just used my intake, exhaust and carb. Anyhow he said he would store my old stuff and I could get it anytime I wanted to.
So I went back to get the sorry old water cooled voltage regulator and all my stuff was gone. He said they had cleaned out the trailer and my stuff must have gotten thrown out. Not taking responsibility for any of it now. I was pretty ticked off but have let it ride. Figured those parts could have sold for $3000.
Anyhow I did pick up a # 4 cylinder head but it is too small to fit my 170. How can I ID what engine it fits?

Thanks,

Glenn