I've done some searching and I can't find anything. The keywords seem too general.
I rebuilt the engine on my new ciao this week because I discovered a crack in the engine case. the next day it stopped running because of an air leak from a crack in the case.
I can figure it out myself but some ideas as to where to look would be helpful: it was along the top front engine bolt both times. Maybe the frame is too close together? Too far apart? I'm going to rule out vibration as that is barely noticeable at most rpms. I'll inspect the frame but I don't think there are any cracks.
for context, it's a small kit (maybe 65cc), stock portmatched cases, and a racing crank... New bearings and seal.