Question regarding what sandpaper to use on my cylinder and high compression heads

So the other day, I got a new engine for my bike. Since they are cheap, I decided to replace the 100cc since its just not running/performing like it should and I can use the 100cc for some spare parts. So the new engine I'm going to have to pull the cylinder and clean out the flash in the ports from the casting. I know its probably on this forum somewhere but what grit sandpaper should I use on the intake/exhaust ports after cleaning them up with my Dremel? I want to properly prepare this engine, check piston is on right way, oil needle bearing, cylinder wall, piston etc. Just need to know what grit to use to give those ports a proper clean up. One other question was I went ahead and bought a high compression head for it. Is that ok to use with it on first startup or should I break it in some on the stock head then switch over to the high compression one? Thanks for any help!

I've never heard of anyone using sandpaper on the ports after dremeling.

The only reason I asked about the sandpaper is because I've heard that after cleaning up the flash from the ports it can sometimes leave a sharp edge the piston can potentially get caught on so had to ask to be sure. And want to use my new high compression head just didn't know with a new engine being really tight on first start up if it would cause damage or too much strain only because engine is new never run yet.