Well - if I am looking at it correctly - there should be a circlip fitted to the outer race of that bearing - shouldn't be able to move far - even with the retainer missing...might be better to split the engine to make sure ?
As for the retainer - I may be able to find one - failing that I can take the measurements to make one...
Do you have any idea what sealant has been used on the cases ???
TZ Parts List
TZ Yamaha Parts & Service - TZ350 METEOR PISTONS IN STOCK - sizes 96 97 98