Easiest is to just have a local bike shop build you a wheel themselves--order the specific hub you want (or re-use what you have), pick the (good) spokes and the (downhill) rim separately, so you know they can't order it already (machine) built.

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Alternately you could build a wheel and use some of the 11ga spokes, as on the Worksman industrial bikes. The spokes can be bought separately from Worksman; they look like moped spokes, about twice as thick as regular spokes. Far thicker than anything a normal bike shop will have. These are for 26" wheels and you will need to drill out the spoke holes in the rim and hub flanges.

There are people who have put pretty fierce 10-15 HP engines on motorized bikes using these spokes, and they still rarely break.
Other parts will break, but the spokes themselves don't.
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