ive got a 84 cr125 as well. if your running 70PSI you most likely need a new top end. Im running around 180PSI with a top end with about 5 hours on it. Doing a top end on a 2 stroke is very easy, i mean very, very easy. As long as you follow the manual's instructions you will be fine. I would reccomend taking it to the dealer to make sure you get the right size piston and get it honed as well. If its a Wiseco piston, you can match up the numbers on the head of the piston so that way you dont have to take it to the dealer to get it sized.
There are a cupple things you should check when taking the 84 apart. you will know what i mean but when you take the cylinder off, you can see a linkage part that sticks out of the crankcase and into the cylinder. These parts tend to break and honda no longer makes them. make sure it ok and that it moves freely without and binding. Also when your finished and your about to put everything back together you might want to check the ATAC chamber and make sure it working properly. if the clutch cover is on, the butterfly valve in the chamber should be completely open, if the cover is off the valve should be completely closed.