I have posted many small tool plans from the book "Metal Projects" Book 3 by John R. Walker before, here are plans for a small two stroke engine and a more advanced 4 cycle engine. The two stroke can be machined from the solid or you can try casting the crankcase block.
When it comes to hobby machine shops, producing small working engines like this, is pure adrenaline.
To download this 13 page file click Two Model IC Engines. 2.1 MB - pdf.