9.75 from firewall to front of cowl.
because the cowl is oblong the dimension from the prop shaft to the plugs (widest dim on DA100) would have to be known.
At 2.5" away from front of cowl you have 12" vertical width but prop shaft will not center in front of cowl. the furthur back you go towards the firewall it gets wider. to keep the prop shaft in center of front cowl you need appx 4.5 to have 12" of vertical usable space
At 6" you have 12 dia so you can orientate the engine anyway you want but at 4.5 the engine would have to mount with cylinders top and bottom (vertical) not horizontal.
Hope this helps so..maybe Jeff can chime in if he has had one ordered to fit that engine.
I plan on using a Moki so at 10" DIA I have no issues at all.