Mowers are an okay start, theyre usually 2 strokes so a bit different from a car but they still have many similar parts.
You might be able to ask around at junkyards/pick a part places if they have engines, especially broken or damaged ones. Even if you never put it together to get it working, you can still learn a lot from just disassembly/reassembly. You could ask local shops if they will let you be an apprentice, maybe make a little money while you learn.
Recommend watching YouTube videos, not necessarily "build videos" since I find those pretty stupid usually, but videos about disassembly and reassembly are important. Cleaning, machining, etc are usually good ones too. Curmudgeon Transmissions is one I like for transmission videos, for example. I like to find the old guys, because theyre the ones with all the years of experience and I find I learn way more from them than from younger guys.