I have the 19 version of that Fox; ran it on a Junior Satan about 40 years ago. When my crude measurements showed the same bolt spacing as the Fox 35 Stunt, I hogged out the mounts and bolted the 35 in place. Yikes! Talk about a nose-heavy model, but it flew. Anyhow, my trusty digital caliper from Harbor Freight just showed the mounting holes as .600 by 1.5 inches for both engines.
An OS 25LA I have shows 14mm by 36mm on the mounting holes, both in the manual as well as with the caliper. That's .55 by 1.417 inches, not too far off the Fox. If you use the OS 25, slightly enlarging the engine's mount holes might work if there's not enough play in the plane's motor mount to shift the screws around a bit. Your choice.
Bob in NEPA