I was in contact with one of the representatives of a Chinese company which provides spare parts for tractor motors and what not - and he said that he had asked his engineers to look for a service manual of this particular tractor engine, but said that they couldn't find one, because the engine is an older model or something. So I got the impression that they do have something like of a service manual. But of course I could be wrong as well. Anyway, thank you for the information!