I think the SB45 could have been built with rails in a way that I'd like, Jack, but in practice it wasn't. The deck or rails had two jobs to do, aside from the elementary ones of holding the frame together and aligned whenever the soleplate was removed, and keeping the engine from falling onto the reel. The first of these more subtle jobs was to restrain the engine torsionally, by providing a reaction to the pulsating torque delivered to the clutch by the PTO each time the engine fired. The second was to withstand the vertical and horizontal vibrational forces created by the engine. Nearly all small single cylinder engines are "half balanced", meaning that the crankshaft counterweight is increased from the size needed to balance the crankshaft itself, by half the weight of the piston, gudgeon pin and the little end of the connecting rod. The reason for half-balancing the engine in that way rather than fully balancing it by applying the full weight of the piston, little end and gudgeon pin to the counterweight, is that to do so would create a horizontal shaking force of the same magnitude as the vertical shaking force created by the piston. Hence single cylinder engines shake like crazy, and the best that can be done without using a pair of counter-rotating shafts or a reciprocating mass like the larger Briggs engines did, is half-balancing and letting the engine, its mountings, and the associated machine also shake like crazy. The mower designer's art, therefore, is to cope with that situation in a way that at least doesn't break anything.
In the case of most vertical crankshaft mowers, the solution is to attach the engine to a quite substantial disk: the mower deck. With a horizontal crankshaft mower, the traditional - especially among the British manufacturers - solution was a set of rather substantial cross-tubes with a bolt through the middle, forming an extremely strong, and given a sensible tube diameter, very rigid, engine mounting. Nobody I can immediately recall but Scott Bonnar chose instead to attach the engine to a pair of very flimsy open channel-sections. It doesn't look as if it would work, and sure enough, it didn't work. They turned an initially fairly competent design into a latter-day Victa 2 stroke: a uniquely Australian design the country cannot be proud of.
