I removed a very old surface mount ceiling light. To my surprise just two wires coming directly from the plaster ceiling ,with no box or ground. The house is appx. 85 years old,is it safe to just connect a new fixture with the 2 wires and replace fixture to plaster again?

This home probably was built with no electrical and it was added later, apparently before some codes were adopted by your city. You could just replace it as is, maybe you are grandfathered over any codes. I'd cut some plaster and install a fixture box. Unless you plan to pull new cable forget about the ground.

OK thanks I really didn't want to start breaking the plaster and lath underneath,

Suggest that you might want to consider using a 1/2" deep, surface mounted "pan cake" box.