The F1 need to be glued, however its easier to mark the holes and the center when its not on the plane.
The distance between engine mounts of OS 46AX is 35 mm, between the mounting holes its 17.5 mm center to center on the same side.
Also the center of the crank (the rod which the prop is sitting on) is at the top level of the engine mounts.

One more thing to consider is the throttle hole in F1, I believe it’s the same as the engine you have if not its not a big deal as long it doesn’t interfere with the muffler.
I would check to see if it’s the same with the engine that you have and use that as a guidance.

The OS 46 AX manual has dimensions at the end on the printed version.

One more thing to notice is that you have enough space between the needle valve and the screw mounting the engine mount to F1 (about 7 mm) so you can reach the mounts screws easly.

I took the height of the engine mount from the plans.

Alex