i have the hangar 9 P-51. i am using an OS-91fx. What is the most reliable and troublefree mounting, inverted or horizontal?

Side mounted. Inverted is fine but more susceptible to screw ups.

warhoover? The only one I know is an S3A/B. Any connection?

Side mounted. Inverted is fine but more susceptible to screw ups.

warhoover? The only one I know is an S3A/B. Any connection?

Any mounting configuration will work.
Os are very reliable engines.

I have no problem mounting a 4 stroke inverted but I prefer to mount 2 strokes horizontal.

Inverted will work just fine, the only problem mounting an engine inverted is that the engine can flood a lot easier seeing how the glow plug is at the bottom. Just try not to prime the engine to much before you start it, you may notice a little richer low end after idleing for a long time. The .91fx is a great engine, if your plane will look best with the engine inverted by all means do it.