I see Stewart Hart has already sent a PM but I'm going to recommend his Pottymill engine, plans for which can be found on the web.
I've just started making a second. The first ran fine, but this time I'm going to be fussy about accuracy and finish. I'm also using the design to improve my Fusion360 skills.
How difficult you find making a Pottymill will depend where you are on your learning curve and what equipment you have. Much easier if you have a milling machine, but not essential. I think all the lathe work could be done on a mini-lathe, the largest part being the 100mm diameter flywheel.
I'm far from talented in the workshop and found building the engine to be a sensible challenge rather than a difficult slog. Making some parts, like the conrod, will have you thinking but mostly the build is straightforward. A very good thing for the beginner is there are few errors in the plan and none that matter.
Dave