Original von Nick50XL
I fear you did actually get it wrong!
The Problem was to get it backwards from 200ccm down to 125ccm!
Now I understand!
I think it should be easier, the main problem should be the inlet port (valvle), it's bigger and different in 200's enginecases.
in 125/150 is like in 200 in the right side, near the flywheel;
the other side, on the left, near the clutch, is different and I think you must keep the 200's side, it's larger than 125/150's side;
the conrod is different, 200's conrod is longer (110 mm) than 125/150's conrod (105 mm) and the piston's hole is bigger (200 --> 16 mm, 125/150 --> 15 mm).
SO you should mount a 125/150's conrod on the 200's cranck.
Than you need something to adapt the bigger 200's hole for cylinder to the 125/150 cylinder, and put on it the new studs cylinder (in 200's engine are 4 x M8*160, in 125/150 4 studs M7*140); you must fix the adaptor in the enginecases.
So you could put a 125/150 ccm cylinder on 200's einginecases. You will have the inlet port (valvle) bigger than a 125/150's engine.
This is only my opinion, of course.
O.T.: I'm in Palermo - Sicilia -Italia