The standard spoke length for the 26" MP5 is 110mm, the same as MPII, MPIII and MP4:
I would use Sapim 13/14g stainless steel single butted strong spokes with 14g nipples, which Sapim recommend for eBikes and tandems:
These butted spokes are very strong
(1250N/mm2) and rated for 1.6 million revolutions.
Gary made
a good video showing how to re-spoke a Magic Pie wheel which may be helpful.
However, I strongly recommend that the spokes are fitted with their heads on the inside of the flanges as shown below:
This provides much better support for the spoke elbows which greatly reduces the chance of breakage due to fatigue and also improves the lateral strength of the wheel.
Check out
this post to see how to calculate the correct spoke length if you are fitting a different depth rim.
Alan