I would lean to a small bar stock engine. Lots of free plans for models that are only 6-8" high. You will have to mill no matter what you do. The alternative is to hand cut and file. You will have to practice hitting the tolerances no matter what you do. So bar stock that can be replaced cheaply would be the way to go. Also, you can repurpose mistakes into other projects.
Most small engines are designed to be made by a lathe only. With a milling attachment, the entire engine can be done. I would take scrap and practice hitting the dimensions within a couple of thousands first Most parts are forgiving, but the cylinder and piston do need to be close. No reason that as you get better, that you can't replace poor parts with better ones.
Most small engines are designed to be made by a lathe only. With a milling attachment, the entire engine can be done. I would take scrap and practice hitting the dimensions within a couple of thousands first Most parts are forgiving, but the cylinder and piston do need to be close. No reason that as you get better, that you can't replace poor parts with better ones.