RE: Reassembly procedure. OS Gemini 120 & 160
There really aren't instructions for assembly and disassembly of O.S. engines. They're all pretty simple, and there isn't any critical reassembly procedure.
The instruction manual for the engine should give the procedure for setting the valve clearances. Basically, though, you rotate the engine so one rocker arm/pushrod assembly is off the cam and set it. Rotate the engine and set the other. You use 0.04 mm and 0.10 mm feelers. The thin one fits and the thick one doesn't. O.S. makes a small valve-setting tool kit, available through your local hobby retailer:
OSMG6334 72200060 GAUGES TOOLS 4-STROKE $10.99
Setting the timing is also very easy. The crankshaft and camshaft gears have small marks. The camshaft gear (the big one) will have a small hole in it, and the crankshaft gear will have a dot. You just insert them so that the dot and hole are in the center and line up with the centers of the cam and crankshaft. They'll all be in a straight line with the timing marks in the middle.
Hope this helps out.