I don't know about the tranny's. I installed numerous 356 engines in VW bugs, a Bus, and a Fastback. They pretty much just bolted in. there were clearance issues with the larger Porsche Diaphram clutch that required grinding away a bit of the bell housing with a dremmel. >
There are 2 very unique 4 cylinder variations of the 356 engine, both "Rare as Hens Teeth". One had dual overhead cams and was the 140 HP Currara C2, with tiny bevel gears and drive shafts that ran the 4 cams. This was only produced in 66, and cost a fortune. They were also prone to problems due to play and slop in the gears that drove the cams. Most C2 owners run a 356 engine and have the rare C2 motor stored for car shows only. The other even rarer engine is the Desmodronic or something close to that. It had tiny connecting rods that drove the valves in both directions, NO VALVE SPRINGS!
Only a couple makes of race cars and Motorcycles ever sported this system. Capeable of Very Very high RPM's due to not "Valve flotation". I have no Idea what the Horse Power ratings were, and have only seen drawings and pictures. They were only built for Racing Porsches, and in limited numbers. These engines could likely run to 12 to 16,000 RPM's. As Horsepower is a function of torque, and time, this would likely put the Horsepower around 350 HP! perhaps a bit more than one would need in an ST4.
I personally would recomend a TDI. They are 90 HP, and I know one will fit in as I had an ST4 with a similar water cooled Rabbit 4 cylinder engine in it. I'll see if I can find any Picts of these 2 Rare engines.