Check comp at WOT, kicking at at least half a dozen times. If your comp falls short, like you say 90~100, then a quick test can see if it's a comp problem.

Get your base figure, and then pull the plugs out and pour in half a tea spoon of 2 stroke oil. Refit plugs and test again. If the comp goes right up to 125 or so then you can be pretty sure it's a ring problem. The oil helps seal the rings for the first few kicks.

If the figure only rises a little bit (5psi or so) then you it's either on the money anyway, or the crank seals are toast, or there's a leak somewhere. Crank seals or a leak can be found with a leakdown test.

Your compression gauge will probably read different to mine and so on, but what you need is a base figure on your gauge on a rebuilt, run in engine. It doesn't really matter what your particular gauge reads unless it's highly accurate, as you want to measure the drop from the base "as new" figure.