The bell for example did not ring at the end of a line - and that 'ding!' is an essential part of any typewriter :-)
The cause of the bell not working, was that the little lever that triggers the bell-clapper stayed 'down'. Similar to the Royal portable machines this mechanism is copied from, the clapper is triggered by a small upright lever that is primed by the right margin stop. This little lever is spring-loaded to return upright for a next end-of-line 'ting' - missing the spring, the bell does not work.
With some experimenting, a spring-shape was bent from a used guitar-string that fits the part and functions to re-set the bell-clapper lever. Guitar-string wire is soft enough that it can be bent, but hard enough to be springy and keep its shape.
The end should probably be shaped a tighter fit around the lever, but it'll do. Maybe it will get some black paint to make it less obvious - or another, improved version will be formed :)
Another issue was the ribbon-reverse. The ribbon transport had already been fixed with an extra improvised spring on the left ratchet, what still remained was the ribbon-reverse push-knobs. The right knob was missing - needing the remaining left knob to be pulled-out to reverse. So sort-of functional, but not as it should be.
These S.I.M. knobs are nicely made hexagonal affairs, with an 8mm head. It turns out that this is exactly the size of a DIN933 M5 screw. To make a new knob, a brass M5 screw of DIN933 pattern was filed down to a similar shape and then threaded over the tip. The original was M2 or thereabouts, for the new, brass knob didn't dare go smaller than M3. As the thread is not visible when mounted, not a big deviation.
The somewhat rough surface of the brass crew was then nickel-plated. This luckily happened to provide a 'matte' finish very similar to the remaining original knob.
Firmly screwed into the now M3 re-tapped bracket inside the machine, it looks credible. And it's functional; pushing the left / right knobs shifts the internal ribbon-transport bracket to change ribbon-direction.
After fitting the repro-knob, it also illustrated why the original right knob could have been lost. Despite being made to the same dimensions as the left knob, it sticks out of the machine much more. It looks easily snagged or be broken off - or just was considered a nuisance and discarded.
To complete the missing parts (at least those known about so far), new feet.
The S.I.M. machine has rubber feet that are held with an inner-flange between two rims on a tube. To replicate that with 3D printing and make it fit reliably would be a bit tricky. So a different design with the machine resting with its the tube-end on the feet-base and a beveled inner flange for retaining only.
The machine must have been knocked-about a bit, because the feet-tubes were not all at the same height. To be able to compensate with the new feet, two different 'sitting-heights' feet version were modelled.
Duly printed in PU-rubber and pressed onto the machine.
The typewriter can again sit on the table by itself. And 3D printed rubber-feet are also grippy enough that the machine stays put when typing - the Synela now no longer tries to sneak off to the left with every carriage advance :-)