Nothing beyond the typical old bike stuff, as you mentioned. The specific condition of the bike you're looking at will be more important that any "model specific" problems.
No, they're not related to the 650s at all, other than being Yamahas.
Dunno 'bout boring them out any. Probably not, would be my guess, as they're not really a 'hot' commodity. But, one never knows what's out there without looking...
Otherwise, they're perfectly nice bikes. They were 'econo' bikes, of course. There were all sorts of variations on the bike - max econo with drum brakes, spoke wheels and deleted electric start; sporty models with F & R disks & mag wheels; cruiser models with cruiser clothes with stepped seat and peanut tank...
The only thing that's 'lo-tech' on them was that they didn't come with counterbalancers like their Honda/Suzuki/Kaw competition did. It wasn't 'till the Seca/Maxim motor of 1983 did Yamaha fit counterbalancers. So, they can vibrate a bit more than the others. Nothing really serious, tho.
I owned a 'Special' model of about 1980 or 81 vintage. Perfectly nice bike, the cruiser version. Rode it for a year or so before selling it. Got great gas mileage, and zippy enough for my old-man riding habits.
Good luck with it. Post some pix if you get it.
Kirk