If you are after the very ultimate in reliability, then a single-engine experimental aircraft with an alternative engine is probably not the right choice.

Whenever you modify something, or build a prototype, little things nearly always go wrong, no matter how careful and skilled you are. That is why the FAA requires a fly-off period for all experimental aircraft. Of the alternative engines, a rotary appears to be the most reliable and robust. However, the typical alternative engine installed in an experimental aircraft has a higher failure rate than the typical certified engine installed in an experimental aircraft.

If your target is ultimate reliability, you should probably buy a certified twin-engine aircraft.

If your heart is set on "rolling your own" (including an alternative engine) then perhaps you should build a twin-engine aircraft using a pair of rotaries. Perhaps a scaled-down version of Rutan's Voyager.