Oscully's right, the space in the triangle is more than sufficient for a 4 stroke, in fact someone else already did a build with one (see photo, not my build and my apologies to the builder I've forgotten its source). He's also nailed most of the issues you'll encounter for a build. The other issues are that the frame is aluminum (which can be more susceptible to vibration damage if the motor is not properly mounted), the handlebar is an odd 1" diameter, the chain guard/fenders have very weak mounts and vibrate easily, and the seat has no springs. All (fairly) easily over come. My comments in my earlier post were more related to how the bike fits a tall / big person. My point was that although it is a big bike, the frames seating dimensions was scaled for an average person. But those issues can be resolved IMO. As for the comment regarding using a 44 tooth sprocket, with the 29er wheel, you'd likely want a bigger sprocket like a 50+ tooth. Add up the costs / time for all these fixes and you'll start to see why I went with a friction drive.
I don't have a link handy, but look for my Genisis Onyx build thread for more photos and info.
I also considered the Firmstrong Chief, which (on paper at least) looks to be an even better candidate for the tall rider than the Onyx: the frame is made of steel, tube diameters are smaller, wheels are 26" and 36 spokes, seating dimensions are scaled more for a tall rider. If there was a Chief locally for me to try I might have gone with that instead. Unfortunately there wasn't and Walmart has a 90 day, no cost to me, return policy (returning the Chief would have been an expensive proposition).