Easiest way, toss a jug and piston back on one side. With a caliper, run the rod of the caliper touching the piston on left or right side of the bore over the wrist pin all the way down to bottom dead center. You'll know you're there as the measurement on the caliper will begin to reverse as you pass BDC. Now. Zero the caliper out with it in the same location. Rotate the crank so the piston goes up to TDC. The highest measurement will be the amount of stroke. 54mm is stock stroke, 58mm is 4mm.
If you don't have a caliper, you can just use a machinists straight edge on the piston. And take measurement off it from the top deck of the cylinder. Just roll it all the way over and make note of the highest and lowest measurements and find the difference. If you don't at least have a straight edge it's not something you should be doing. Pick one up at a tool store for cheap.
If you get 58mm, take note if there was a shim plate between two gaskets at the base of the jugs, or if the head has work. Also take note of all the markings on the side of the Rod. CK158 would be a 115 rod and you need a piston with a different pin height to work with it.