I daily drive a 2003 Honda Civic EX Auto with 140K miles on it. A few months ago, after driving for 40 minutes, I heard a thud, and the whole car was enveloped in a thick cloud of smoke, but it kept going, but with very little power, wouldn't go above 30mph

I was able to get it home, and I got an offer for a replacement motor from a friend for $300 so I didn't bother tearing into it until recently.

I tore apart the motor and other than a burnt oil smell, nothing stood out to me, the crankshaft and rods look immaculate, the bearings were all a little worn, but normal wear for 140K mile car, the piston rings had more than normal blowby, but just enough to cause the occasional misfire. After I got it home it would barely start and idled really rough. The head gasket seemed okay but I'm not very good at telling what's wrong with those. 

Any theories on what may have caused it? Thanks