The first thing we have to ask, have you any experience of making more than two bits that fit together?
Can you read a drawing and make a part from such a thing?
If the answer is yes to both of these, then you will have no trouble making an engine.
But if you are a bit of a beginner, I personally would go for a little oscillator.
There are three reasons for that, the first being is that it doesn't take too long, most probably a weekend for a basic type. The second is that if you can't get one of those going, you are in trouble, and the third, when you do get it running, it gives you such a buzz, you feel like you can take on the world.
Then you make some sort of intermediate engines, before progressing onto something that uses fuel and sparky things.
Basically, start easy, and progress gently from there.
Too many jump in with both feet projects end up in tears, either ruined castings or just lose interest because it is too difficult.
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