OK, things become a bit clearer. The O-ring under the starter may not apply to your engine (I have only a vague idea what a series 70 is) but it is an unreliable component on the engines that have it, and it does cause an induction leak. The other possible cause of over-revving is that some activist may have messed with the part of the carburetor that operates the governor. If you have an induction air leak you would probably have hard starting and/or loss of power.
The reason for your loose cup is the cracked flywheel. You cannot tighten the retaining nut properly because the crack just opens up and lets the flywheel move down the taper. The only way to improve the situation is to replace the flywheel. The loose cup is probably also the reason for the clattering noise you complained about. The belt isn't likely to cause a noise like that.
Don't get too enthusiastic about a new four stroke. The affordable ones mostly have poorly-made engines that might make you long to have your Victa back. A flywheel change might result in a complete cure of everything you have reported. And it will give you a chance to smear a little grease on the breaker-point cam, if you forgot to do so when you reset the points.
