Nice job, did not realise it was this problematic to source a rev counter for the little car.
Maybe this will help someone: type 'obrotomierz Lumel' into the search engine of your choice. There is a few models, see pics. I have got the round one, there are also two flat shaped ones (left side of the second picture), a dial one and a LED one. One in good nick should be no more than £40, but it is possible to get them for less. The flat ones must come with their 'foot', they are dashboard mounted with screws.
The round one is adjustable between 2 and 4 cylinders, not sure about the other two, but I would imagine they will most likely be as well. Practical reasons, the parts were not just available everywhere off the shelf back then, so the manufacturers' approach was a bit more universal.