For me the Trojan suspension is well founded but ultimately fails to cope with the practical demands made of it. On the basis we are looking at larger diameter wheels for comfort and can use modern tyre technology to achieve a narrow but grippy tyre the larger wheel can be offset against width (Modern manufacturers note), meaning the width need need breach microcar proportions to provide safe yet lively performance. 10 inch minimum and possible 12 inch. So steering bars are probably out, for modern use, save that the Heinkel rack could stand gearing up a bit. The Goggo front suspension works well, as does the kingpin in needle nardle noo bearings. All it wants is a tweak to keep water out. Adjustable coil overs on here and you have a pretty tunable system that will cope with most situations, rally proven in Australia and Africa. Nick the disc brake option from the NSU Prinz and stopping is no issue.
The above is not fixed to width beyond that about 4 inches narrower than a Goggo. We approach a Messerschmitt type front end with better suspension, steering and brakes.
Rear needs to be more akin to the Tiger arrangement but with a lighter lower wishbone plus a second wishbone to control the camber of the rear wheels better. Effectively like a small Formula Ford or other racing set up. The best period engine is a more difficult selection. Clearly the wankel is best but is it microcar? Probably not. The NSU 600 is a bit big for what I had in mind. However the Steyr Puch offers low centre of gravity and great tune-ability. It matches Fiat gearboxes, and if there is a budget a 5 speed gearbox is achievable. So that gives is roughly a Fiat 500 width rear.
Taking the front and rear, borrowing from Gordon Murray and precursors, it is then possible to place a bench seat behind the drivers single seat. This offers seating for one fatty, two normals or three kids, or a large amount of shopping. In most cases the car retains its neutral weight bias for cornering allowing for efficient fast a to b speeds without lots of point and squirt. The front to rear is neccasarily rear biased but remember the driver is sitting much more forward than in most cars. Much of bthe ancillary gubbins can be housed in the nose further evening out weight distribution. However should the car loose adhesion its Achilles heel will be the dumbell nature of weight distribution over a mid engined car. So a spin would last longer. To alter that to mid engine might mean to long a wheelbase for the track. Only some complex maths would prove some kind of sure guide.
Body, effectively a teardrop traveling backwards with aerodynamic wings guiding air to two intakes, but allowing the passingers to close into the car for access. So a car with a waist. Probable an asymmetric gullwing door/roof structure with a pop out window on the opposing side for summer driving and emergency escape. This would mean left and right handed versions probably, so if theoretically productionised, that might have to alter. However it would be a four wheeled bubblecar and would singe the arse off a Tiger for a little more width and a huge dollop more of practicality. All parts were available at the time, it could have existed.