I think the problem with all of these is A) the tooth spacing B) the general stiffness of the material and C) where and how do you attach the styli?
The puny weak electric motor and plastic bearing surfaces the capstans rode in necessitated a belt that was pliable/flimsy by nature so as not to create a high load/tension on the moving parts.
Using the Makita belt as an example this belt is used on a machine that has Steel pulleys(early models, now aluminium) ball bearings and has a 16000rpm no load speed so has to be stiff and sturdy as a result. Just placing such a belt on the printer capstans is likely to tear away the bearing mounts due to the tension and there is no way on earth the motor would even turn such a belt.
All joking aside I know the German lads especially have put a lot of time into this and would like this to work,so as to restore an important part of Sinclair history and I applaud them for doing so.
But the fact remains that again all humour aside there is no paper supplier no styli supplier and this printer also if memory serves is fitted with a ULA and we all know how fragile they are.
If in the end you can find some one who will supply all of the above for the twenty or so people in the world still using this over complex abomination at a COST EFFECTIVE PRICE then I might be tempted to resurrect my half dozen pieces of junk, until then it's Alphacom all the way for this USER as opposed to a COLLECTOR of ZX goodness.