Stihl makes a concrete saw--model 710. It's 110cc single and it's a BEAST. But, once you get it out of the saw, it's actually very light.

They get droppeed or run over by machinery. Engine is still good, but the saw will have a broken arm or the case is mangled beyond repair. I got mine for $50.

You'll have to cut the engine out. It's part of the saw housing, just like some of the chainsaw engines.