There is not a clever answer, sadly. I give you my view.
The engine has no effect on the rear suspension other than a slight force as it powers up, tensioning the chain vertically, so no prob. The carden shaft is the key element. The engine can vibrate but the shaft remains in drive all the time -Spring to the Swing- remember. No energy sapping girt rubbers. No engine solidly mounted to chaindrive so the whole thing bobs up and down on mounts and suspension. No collars to wear on a rubber mounted engine like the Heinkel, which still weaves on its mounts anyway. The schmitt does not offer engine vibration, try the KR175 and you get what one other option offers using pretty much the same unit. So much vibration that the regulator regularly falls off or fails!
That leaves the swing arm. Given the choice of hardly any suspension or a suspension system. Bring in weight issues, we want it to be light weight, then two big rubber in torsion units is more than adequate. At 50 odd years of age these units are at the end of their life. Pretty much like many other suspension set ups, actually. Since the rubber unit is the suspension, no bushings or other joints, shackles etc. you can but replace it. With new rubbers on the deflection on cornering is acceptable for the performance of the car. It is predictable. If you tune a Schmitt up and are looking at high speeds then you can buy a long available extra frame that helps locate the swingarm with a mounting on the chain case plate.
In fact most movement on the rear is deflection of the tyre used. That is why you use the best tyre you can buy on the back. Put a cheap trailer tyre on there, even if it is 6 ply, is not going to cut the mustard. Some have gone further and put in a wider section tyre with solid sides on 'panzer' wheels made of thicker steel, some wider.
I would put a Schmitts directional stability and maneuverability at speed far in excess of most of the competition, if all in optimum condition, but only with good tyres on. Where it is less good and eclipsed, is in slow handling, as the steering has a large turning circle and requires creep to turn without major effort on the steering bar. It is also a much harder ride overall and one very good reason some people reject it for the softer ride of other cars. Not everyone wants to drive like a loony.
As a separate test, look at the speed and ease with which you can get the engine out. The MOC have run races in the passed and I believe some ridiculously fast times were recorded, but 20 minutes as an easy target. Bonds step forward. The car remains mobile without the engine. Its well thought out.