Yes, as Bretto said, I think you will have to feel for it....and even at that there will be a small amount of crank rotation that will result in little or no vertical piston movement, when it's right at the top....but you can average it and find center, and you will get very close...and with that mark on the crank pulley you should be able to find it without too much trouble..
Then check your flywheel, and scribe a mark if you can...or maybe if you are lucky one of those numerals in the part number will line up!
But, it runs now, right?
If you were to put a continuity-tester across your points, and rotate the engine until the rotor is pointing at #1 wire position, and when the points break, then you will know where the present setting is.....unhook the battery though, you are just looking for continuity....then compare that with all your various marks, and maybe even the wire-feel test..