I don’t have any direct experience with the Mini Swift yet, but since you said it is a new motor, could it be the clutch?  Could the clutch be engaging too soon, before the motor hits the torque curve?  I had a similar issue with a Gazelle with a brand new clutch, that it was engaging too soon.  Just something to check if you haven’t already.

I took it to our local kart shop, and he put it on a lathe and removed some of the material.  IAME clutches are known to engage a little early on the lower horsepower motors, and typically the clutches should be cut some prior to their first use.

Once we got our clutch cut, the starts were much better and my daughter didn’t bog down and get passed so quickly when the green flag waved.

Spray the clutch down with WD-40 and see if it takes off better.  Otherwise your ignition timing may be off.