have you tried a thicker base gasket to see if maybe your squish band is getting hit by the piston at tdc?
In order to see if it’s indeed hitting the head, put spacers on the studs between the cyl and nut and tighten down with no head.
If it doesn’t stick then you are hitting the head and need to add head gasket or add base spacer.
My bet is that you’re hitting the head or the new seals and bearings are making it hard to spin over. Did you lube the seals during installation?
As well, is this a 70kit or a 50cc? Orig stock head with a 70cc kit will hit for sure without modding the head or spacing it out a bunch (edited)