If you really want stout, based on some of the results I've seen so far here on ES, you're probably gonna have to make it yourself.
There's some good freewheels out there, as evidenced by the stuff in the "destroyed" freewheels thread, but lots of bad ones, too.
Same thing with the actual cranks with the threads to mount them on. Some have actually broken off when used to pedal, according to at least one ES thread (with pics) that I cant' remember teh name of right now.
Since you definitely ahve the skills to do it, and tools, I would recommend a combining jackshaft, that merges crank and motor power at a separate point, so you don't need anything special in cranks, and you can use whatever type of freewheeling mechanism you wish for them and the motor, separately, on the jackshaft itself.
Something like this is the plan on mine, but mine will have a NuVinci as not only the jackshaft itself (doublefreewheel on it's right side input) but also the pivot point for the suspension (which yours doesn't have).
Alternately, just run it the way Rassy did on his trike, "under" the freewheel on the motor instead of over, and use whatever freewheeling cranks you want.
You've got plenty of room in that frame for about any way you want to do it, though.