Lawn mower engines are made of cast aluminum and some lawn mower engines also have steel inside the cylinder for extra strength.
So if you have an old aluminum lawn mower engine that you want to recycle you should check the cylinder with a magnet and if it sticks it's steel inside the cylinder but the rest is aluminum.
Some lawn mower engines are sometimes aluminum cylinder bore engines as well without the steel in the cylinder.
These engines are also very reliable engines even without the steel but the lawn mower engines with the steel insert are often longer lasting.
Years ago though some lawn mower engines used to be made of cast iron.
I have an old Briggs & Stratton lawn mower engine that is made of cast iron and I bought it at a yard sale.
It wasn't running but I got it running and mainly just use it to play with and start and run it once in awhile and take it to shows.
If you have a lawn mower engine that is not broken down and any steel is not removed then the lawn mower engine is dirty aluminum at a scrap yard but if you can break the lawn mower engine down and remove any steel, seals, bolts etc then it can be clean cast aluminum.