If I remember correctly the 1.5 is a 5 bearing crank and the 1.8 is only a 3 bearing crank so the 1.5 will be the sturdier engine, not that they are highly stressed in a broads cruiser, I doubt the extra 300cc has much benefit on the rivers.
Surely it didn't eat the bearings overnight, that would only happen while actually running but you said it was running fine when you turned it off, would be noisy too.
One of the common problems on the stuff I work on is cam failure on low hour engines due to condensation causing corrosion on the lobes but it's normally only a couple of lobes not all at once and a solid tappet engine would get very noisy first.