I agree with Michael G. A seemingly simple engine built scratch from castings is quite a task.
Even refurbishing an existing engine can be trialling. I bought a Russian made Mills diesel, a copy of the famous 0.75cc produced in the UK many years ago. I was into RC Old Timers before my recent foray into 5" gauge Queensland railway engine. (AC Neilsen A-10)
It had never run for the previous owner due to the piston/liner fit at TDC and other reasons no doubt. Was extremely tight. Well, I bought some diamond lap compound and lapped the piston to bore with a nice "nip" at top dead centre, checked the timing, ports seemed to be exposed at the right time, fuel flow, jets and not even a kick out of the thing….ever. (Fresh fuel and all) Ebayed it as I was thoroughly sick of the sight of it! I'm sure someone smarter than I could have got it going.
If looking to refurbish, acquire an engine that has been a known runner, glow or diesel.
Cheers
Andrew