You may have accidentally snagged the crank seal while reassembling. Be very careful when mating the crankcase halves so you don't knock the gasket out of place.
I had one of those bikes when I was in high school. I was a fair bit too big for it, but it ripped. I bought mine, in need of a top end, for 100 bucks. It was the best smile per dollar bike i've ever owned. It would flip over in top gear with my tall skinny arse on it. I gave several people a good laugh with that little death trap, lol.