You have to get more-or-less "engine specific" bearings. I dont know why, but maybe they are made differently to general-use bearings?? They have clutch specific bearings e.g: 5x10 bearings - they have 5x10 clutch bearings and 'normal' bearings.

EDIT: Oh, I also got a ceramic rear bearing through Acer for a TRX 3.3 I have, it wasn't an "engine" bearing, but it did very well and never had any issues with it, just removed the shields on it and installed it. Im sure it would be fine to get it and put it in your engine. Just keep an eye on it from time to time.