Your comments are quite helpful, especially in light of other postings with respect to the kit:
1. "The kit uses a red LED. That gives less light than an orange-yellow LED because our eyes are less sensitive for red. I think that I am going to change it."
I can see that optimum use of some alternative LEDs might require some modification of the LED modulator and operating voltage.
2. "The diffuser is far too transparent. The center of the picture is much brighter than the edges. I will look for a better diffuser."
The "workload" of the diffuser is increased by the use of a single LED against the relatively dark background of the LED circuit board. I think one could surround the single LED with a small reflector array that would result in more uniform illumination of the image field, perhaps making the choice of the diffuser material less critical. Of course a small array of even four LEDs would also help, but that puts us back to #1 and some mods to the LED driver.
3. "The picture synchronises good, but with the sync bar exactly in the middle of the viewing window. I don't know yet how to change that. The framing can be shifted with the speed control, but then the synchronisation is less stable and tends to speed up the disc to too high speeds."
If the sync bar is horizontal (a line sync phase error), a slight lateral shift in the position of the optical sensor should eliminate the problem.
In any case, if I didn't enjoy tinkering, I would buy a television set and not a televisor!