Here's the first attempt at putting both ICE seat frame and steerer assembly onto the Kettwiesel frame. What quickly becomes apparent is like this, the chain line between crank and rear cassette would run straight through the base of the seat, and its support.
More pleasing is seeing the new aluminium MTB handlebar working well with two titanium "uprights" (that were left-overs from the original Hase hand-trike.) Researching the different varieties of Kettwiesel that have been made over the years, single front ring Kett's range from 39T through to 52T, which would result in a slightly different front height for the chain run, depending on what ring I choose. I'll need to provide some leeway for the route in case I need to change the size of the front ring. I'd like to solve the problem by modifying the seat support so the chain (tube) can clear all obstacles, and run unimpeded, rather than having to add an idler wheel.
First thought is to widen the distance between the ICE seat frame supports, and increase the height between them and the top of the boom clamps. The chain could then run through the gap this creates.
I'll mount the bottom bracket and crankset as a next step, then set the boom position to roughly what I'll be needing - some trial and error with the chain in place will reveal what to change with the seat clamps, and also whether the return route for the chain will clear the steerer's handlebar.