I haven't posted more information here recently, because of issue with forum, but as now is fixed I will continue.

So logically after engine came clutch and gearbox.

As the plan is for big increase of power and also general opinion is that gearbox is not so strong let say I start to think for options in advance.

And some time ago I bought Scara73 Gr.N gear kit consisting of 1-3 gears and main shaft
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These gears have wider tooth base than original, i.e. less number of teeth. Here is a comparison photos made by another guy, who have use that kit (nickname Stichl), which I have found. On picture below are showed original and Scara73 3rd gear wheels as original ones are on left.
3rdGear.jpg
Also needle bearing (if that is correct name in English) on 3rd gear is different as can be seen from below picture where original one is on the right.
3rdGear_flange.jpg
Main shaft is thicker and also splines. Original one is at bottom on below picture.
MainShaft.jpg

After some time I started thinking that will be better with 4rd and 5th gears also if possible and we contacted Scara73 again and they reply that it is possible to be "upgraded" that kit and there is options for 5th gear ratios only. And gears were ordered with 5th slightly taller and 4rd is approximately the same ratio no option there. At same time was ordered from them rigid 5-puck ceramic clutch disc with splines for thicker main shaft. And all gearbox's bearings was changed with new one to be as new.
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Gearbox casing also need to be reinforced. Originally there is one plate on one side of gearbox. Custom "spectacle plate" was made to prevent both shafts from splitting (below is not final state).
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Final drive is kept original one.