Zach:
Some older bikes pistons didn't have the intake boost hole carved on the piston. I know as a fact that the 91 KX125 didn't have that hole. You have to make it or find someone who can. The top bearing and wrispin is the same, the height from wristpin to crown is the same. It has a flass head vs an small convexity found on later models. This helps to keep the compression ratio low.
When you study the important parts on the piston geometry, besides the right bore, you'll find that the skirt lenght is important to close the exhaust port when the piston is at TDC, the window to open the rear booster port geometry, the side cutouts not falling into the exhaust window.
As I said, look over the WISECO spec page on piston kits and see your options. You must find a 56.5 mm dia with the stroke higher than 53. When you check the numbers for the top bearing, make sure are the same for your bike year model. Also the exhaust valves have to be grounded down to maintain 1 mm offset with the bore.