Engine rebuild question

[shinigami710]

Tried searching but couldn't come up with a definite answer on it. My engine is pushing 40k and I have no idea how the original owner was about maintenance. Im still riding and its ok for now but if I wanted to rebuild it in the future I ask these questions. A) what all should I replace? I've rebuild car engines but never bikes. B) where would I get said parts easier? C) while I've got it tore apart is there anything I can do to juice it up a bit internally? Raise compression, etc. any help is much appreciated.