How do I check timing? Any special tools required?

No special tools besides a timing light.

I was reading about a piston stop tool to set initial timing. Reman powerhead just installed.

I have a stop that I made from an old spark plug. With the center electrode removed drill and tap from the side for a set screw. I think a long 5/16th bolt will work through the center for a stop. Rotate to stop both directions middle is what you want.