Re: 48cc vs. 69cc.
Yeah, you shouldn't have bearing problems, but the rest of it seems common enough.
Fenders have to be beefed up with stronger supports and elbow brackets. The chains can be rid of the master links and put together without them for a stronger hold (that's what I did).
Stripped hardware is common because it's made with American scrap metal filtered through Chinese noodles so yeah, it strips easy.
The clutch lock pin.....you mean on the clutch handle? Those clutch handles are junk. Expect them to brake or crack, and give you a headache.
Basically, any of these engines should be upgraded with better parts.
Don't feed the bears....