well; that doesn't sound good at all!
Let me start with
5: Your Estrella must be a B1 or B2, so a imported from Japan.
The German Estrellas have a different carburettor! With only 17PS (HP), different needle and head. There are surely different part numbers, too! No way to help you!
But maybe Rodorinx or a Japanese reader can???
I think that all other items should be the same in the early Japanese B-versions and the German A-version. The A2 came out in 1994, 2 years after the start of the B1 in Japan.
I can send you a CD with pictures and part numbers on it. Don't know if this will help?!
You surely have Kawasaki dealers in Singapore. Can't they help you? If you need a new piston (German = Kolben) and / or piston ring, a Kawasaki dealer or a well equipped workshop should do the work! Same with the cam (German = Nockenwelle)!
But 50 kmph sounds real bad!
"Der Gelbe Ulrich" from the Netherlands bought an exchange piston some weeks ago. But the bore has to be enlarged a bit and that's really tricky! The main Kawasaki stock for whole Europe meanwhile is in the Netherlands.
But I think it causes big problems to buy them in Germany or the Netherlands! Is there no way to get the parts at a Kawasaki dealer in Singapore? Maybe you try to get into contact with shops in Hong Kong?!
Well; the more precise you ask or describe your problem, the better we can answer.
(and wishes for reanimating your Estrella)