Most likely it's not the engine, but something else like a wheel bearing or the clutch. My little cheeftah makes some strange noise when I get in the high speed range, but most of them are from other parts rattling.
If your questioning where the noise is coming from, set the bike on a milk crate and rev it up to top speed and then you can get a better idea where the noise is coming from.
Just have someone with you to steady the mini so it does vibrate off the crate.