Here's an old Boy Scout skill. Use a stick to measure something whose size you don't know by comparing its projection to something you do know. In this case I used a pencil to compare the head tubes to the fork blades. I don't know how long fork blades are, but it's safe to assume they're almost identical. You could do me one better by using a ruler. Or compare it to the rim diameter, 622 mm.
Or you could eyeball the proximity of the handlebar ends to the top tube. In the case of the Victeur, they're about level with the lower edge of the top tube. In the case of the Planet X, they're a lot closer to the down tube. You can flip the stem and fiddle with spacers to bring the bar up to the same level on the Planet X. Or a cm or two lower, allowing that the Planet X is larger and has a longer head tube. Of course, this stem might still be too long. Back in the day, we used to approximate reach by measuring the distance between the saddle nose and the handlebar with our forearm, elbow to fingertip. I think the biblical term for it is "cubit." My span from saddle nose to handlebar center is 1 cubit plus 8.75 cm. I ride with a very flat back.
I know this isn't as accurate as a factory geometry chart, but you don't have that, so you have to improvise, and the proof is in the riding anyway.