Engine & Internal - smoking 91 civic wagon help

my car started smoking yesterday after a longer drive. 1.5 liter motor with 101k. smokes when downshifting and in 5th gear when accalerating. did it yesterday in the mountains (about 7-8 thousand feet) but cannot recreate it today. any suggestions on the the problem??
if i needed a new engine, how much would a decent used one cost?? how much to install?? thanks

At any time during the long drive when it was smoking did it overheat, or is it overheating now?

No, the temperature never got close to overheating, if fact it was early in the morning and it was quite cool. any other thoughts??

i know my car is smoking to...it could be your valve guides of valve seals. u should also do a compression test.