Motor Questions from Newbie

I have the 494 non rave that is suppose to have 12 races on it. I use to race dirt
bikes with two strokes and I rebuilt the top end every other race. What is the norm
for these engines?
I have read some places on the forums that most people do a compression test to
decide, if so what is the expected psi for a national motor?
And what is the exact procedure, I have never done a compression test before.
Also what is the standard policy of parts to change?
And where is the best supply house?
Also how does everyone take care of the valves? How often to replace them.
I also I believe I seen on the forum somewhere that someone had a video of how
to do a top end rebuild and if so I would love to have one.
My email: johnnewcomb62 at yahoo dot com If whoever has one would please
email me so I can get one from them.

Thanks in advance
John