for replacing a drive shaft seal yes. But for the shift shaft seal the cases do not need parting. Remove the clutch, pull out the shaft and the seal can be popped out from the inside.
I have replaced many seals using self tappers and it works fine.
Hi and welcome to OSS Yann.
Didn't MIG go tits up some 10 years ago?
http://www.moto-net.com/actualites-motos-458-Business-DOMMAGES-COLLAT%C3%89RAUX-MIG-met-la-cle-sous-la-porte.html
haven't done it myself on an oilcooled engine (yet) but I'd guess you should take care not to push 26 too far in because it could disengage from 6 or the spring 27 could unhook from 28.
No.
You mentioned fuel leaking from under the carbs in your first post. Could be float needles on carbs 1 & 4 leaking, and overfueling those cilinders.
instead of trying to make the aircooled lump perform better, it would be cheaper to just get an oilcooled engine and fit that. You say they are hard to find in Sweden so try looking abroad, Finland for example.